I got rid of some errors by removing the required annotations from things that are not populated via the form like UserID, but I still have the problem of category specific required fields being required no matter what category is selected. Those fields must be required whenever a record is inserted into a category specific table, but not when a record is being added to a different category.

These are some code snippets from the relevant parts of my page:

Here is an example from the Services model

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace PostAlmostAnything.Models
{
    public partial class Services
    {
        [Key]
        public int Serviceid { get; set; }
        [Required]
        public int? Post { get; set; }
        [Required(ErrorMessage = "Price Required!")]
        [Range(.00, Double.PositiveInfinity, ErrorMessage = "Must Be Positive Number!")]
        [Display(Name = "Price:")]
        [DataType(DataType.Currency)]
        [Column(TypeName = "decimal(18, 2)")]
        public decimal? Price { get; set; }
        [Required(ErrorMessage = "Service Rate Required!")]
        [Range(1, Double.PositiveInfinity, ErrorMessage = "Service Rate Required!")]
        [Display(Name = "Service Rate:")]
        public int? Servicerate { get; set; }
        [Required(ErrorMessage = "Currency Required!")]
        [Range(1, Double.PositiveInfinity, ErrorMessage = "Currency Required!")]
        [Display(Name = "Currency:")]
        public int? Currency { get; set; }
        [Required(ErrorMessage = "Service Hours Required!")]
        [StringLength(500)]
        [RegularExpression("^(?!.*<[^>]+>).*", ErrorMessage = "No HTML tags allowed!")]
        [Display(Name = "Service Hours:")]
        public string Hours { get; set; }

        public virtual Currency CurrencyNavigation { get; set; }
        public virtual Posts PostNavigation { get; set; }
        public virtual Servicerate ServicerateNavigation { get; set; }
    }
}

Then the form used to create a new post loads partials like this when a category is selected:

 <div class="form-group row">
            <label class="col-sm-2 col-form-label" asp-for="Posts.Category"></label>
            <div class="col-sm-7">
                <select asp-for="Posts.Category" class="form-control" asp-items="ViewBag.ListofCategories">
                    <option value="0">Select Category</option>
                </select>
            </div>
            <span class="text-danger col-sm-3 col-form-label" asp-validation-for="Posts.Category"></span>
        </div>

<div id="categorypartial" class="form-group">

        </div>


$('#Posts_Category').change(function () {
                var url = '@Url.Content("~/api/")' + "Categories/Getsubcategory/";
                var ddlsource = "#Posts_Category";
                $.getJSON(url + $(ddlsource).val(), function (data) {
                    var items = '';
                    $("#Posts_Subcategory").empty();
                    $.each(data, function (i, subcategory) {
                        items += "<option value='" + subcategory.value + "'>" + subcategory.text + "</option>";
                    });
                    $('#Posts_Subcategory').html(items);
                });
                $('#categorypartial').load('/post/' + $(ddlsource).val() + '?handler=CategoryPartial', function () {
                    // Remove previous validator
                    $("#newpost").removeData("validator");
                    $("#newpost").removeData("unobtrusiveValidator");
                    $.validator.unobtrusive.parse("#newpost");
                });
            });

Then in the Index.cshtml.cs file I have this

[BindProperty]
		public Posts Posts
		{
			get;
			set;
		}
		
		[BindProperty]
		public Community Community
        {
			get;
			set;
        }
		[BindProperty]
		public Housing Housing
        {
			get;
			set;
        }
		[BindProperty]
		public Jobs Jobs
        {
			get;
			set;
        }
		[BindProperty]
		public Personals Personals 
		{
			get;
			set;
		}
		[BindProperty]
		public Rantrave Rantrave
        {
			get;
			set;
        }
		[BindProperty]
		public Services Services
        {
			get;
			set;
        }
		[BindProperty]
		public Shopping Shopping
        {
			get;
			set;
        }
		public InputModel Input
		{
			get;
			set;
		}
		public IActionResult OnGet()
		{
			ViewData["ListofCategories"] = new SelectList(_context.Categories.Where(c => c.Site == GlobalStatic.SITENUMBER()), "Categoryid", "Categoryname");
			return Page();
		}
		public PartialViewResult OnGetCategoryPartial(int Categoryid)
		{
			string partialname = String.Empty;
			switch (Categoryid)
				{
					case 1:
						partialname = "_CreateCommunity";
						break;
					case 2:
						partialname = "_CreatePersonals";
						break;
					case 3:
						partialname = "_CreateHousing";
						break;
					case 4:
						partialname = "_CreateShopping";
						break;
					case 5:
						partialname = "_CreateServices";
						break;
					case 6:
						partialname = "_CreateJobs";
						break;
					case 8:
						partialname = "_CreateRantrave";
						break;
				}
			return Partial(partialname);
		}

Here are the parts of the post event relevant to Posts and categories

public async Task<IActionResult> OnPostAsync()
		{
			if (!ModelState.IsValid)
			{
				return Page();
			}
			// Set Parameters That Don't Require User Input
			Posts.Adminban = false;
			Posts.Active = true;
			Posts.Dateposted = DateTime.Now;
			Posts.Site = GlobalStatic.SITENUMBER();
			Posts.Ipaddress = _accessor.HttpContext.Connection.RemoteIpAddress.ToString();
			int? sc = Convert.ToInt32(HttpContext.Request.Form["Subcategoryid"]);
			int? sc2 = Convert.ToInt32(HttpContext.Request.Form["Subcategory2id"]);
			Posts.Subcategory = sc;
			Posts.Subcategory2 = sc2;
			if (User.Identity.IsAuthenticated)
			{
				IdentityUser user = await _userManager.GetUserAsync(User);
				Posts.Userid = await _userManager.GetUserIdAsync(user);
            }
            else
            {
				return Page();
            }
			_context.Posts.Add(Posts);
			if (Posts.Category != null)
            {
				int? catid = Posts.Category;
				switch (catid)
                {
					case 1:
						Community.Post = Posts.Postid;
						_context.Community.Add(Community);
						break;
					case 2:
						Personals.Post = Posts.Postid;
						_context.Personals.Add(Personals);
						break;
					case 3:
						Housing.Post = Posts.Postid;
						_context.Housing.Add(Housing);
						break;
					case 4:
						Shopping.Post = Posts.Postid;
						_context.Shopping.Add(Shopping);
						break;
					case 5:
						Services.Post = Posts.Postid;
						_context.Services.Add(Services);
						break;
					case 6:
						Jobs.Post = Posts.Postid;
						_context.Jobs.Add(Jobs);
						break;
					case 8:
						Rantrave.Post = Posts.Postid;
						Rantrave.Nameurl = linkgenerator.rrtagurl(Rantrave.Name);
						_context.Rantrave.Add(Rantrave);
						break;
                }

            }
			await _context.SaveChangesAsync();
			string targeturl = linkgenerator.postlink(Posts.Postid, Posts.Title);
			return RedirectToPage(targeturl);
		}

So there it is. For efficiency I did not include all of the model and I limited the code for the dropdown lists to just categories because the partials loaded based on category on the ones with the required fields. 

The problem seems to be that every partial is being required regardless of which one is actually loaded into the form. The solution to this is probably simple, but I am new to .Net Core and have never built a form this complex before. Even after I fix this issue I still have to add a feature for uploading images.

UPDATE: I made some changes. I removed the BindProperty for all the category tables in my index.cshtml.cs file and I moved await _context.SaveChangesAsync(); up so that it runs before I need to grab the Postid of the new record. Now I get the following error:

Unfortunately, When I try to post to Community I get an error saying that

An unhandled exception occurred while processing the request.

<div class="titleerror">NullReferenceException: Object reference not set to an instance of an object.</div>

PostAlmostAnything.Pages.post.IndexModel.OnPostAsync() in Index.cshtml.cs, line 177

• Stack 
• Query 
• Cookies 
• Headers 
• Routing

<div id="stackpage" class="page">

• NullReferenceException: Object reference not set to an instance of an object.

  • PostAlmostAnything.Pages.post.IndexModel.OnPostAsync() in Index.cshtml.cs

    <button class="expandCollapseButton" data-frameid="frame1">-</button> <div class="source">
    171. await _context.SaveChangesAsync();
    172. if (catid != null)
    173. {
    174. switch (catid)
    175. {
    176. case 1:
    177. Community.Post = Posts.Postid;
    178. _context.Community.Add(Community);
    179. break;
    180. case 2:
    181. Personals.Post = Posts.Postid;
    182. _context.Personals.Add(Personals);
    183. break;

    So, why isn't scoped identity working as described here https://www.entityframeworktutorial.net/faq/how-to-get-id-of-saved-entity-in-entity-framework.aspx

    </div>

</div>

According to this https://entityframework.net/retrieve-id-of-inserted-entity what I am doing should work because the record is created the same way right before the Postid is needed and used. I checked the database and confirmed that the new record was created and the new record has a Postid, but Posts.Postid is null according to the page.