Hi Aliyilmaz,

Your onclick event does not put the id you want to pass, here is my demo:

Model:

 public class Cat
    {
        public int Id { get; set; }
        public string KategoriAd { get; set; }
        public ICollection<TProject> TProjects { get; set; }
    }

 public class TProject
    {
        public int Id { get; set; }
        public string ResimYolu { get; set; }
        public string Baslik { get; set; }
        public string İcerik { get; set; }
        public int CatId { get; set; }
        public virtual Cat Cat { get; set; }
    }

View:

<section id="td_work" class="td-work">
    <div class="container">
        <div class="d-flex justify-content-between mb-60">
            <h2 class="section-title color-1 mb-0">Biten <span>Projeler</span></h2>
            <div class="controls">
                <button type="button" class="control" data-filter="all">Hepsi</button>
                @foreach (var item in ViewBag.Pcat)
                {
                    <button onclick="GetProje(@item.Id)">
                        @item.KategoriAd
                    </button>
                }
            </div>
        </div>
        <div class="filter-items">
            <div class="row">
                @foreach (var item in ViewBag.Proje)
                {
                    <div class="mix col-lg-4 col-md-4 col-sm-12 col-12 @item.CatId">
                        <div class="box-item">
                            <h3 class="item-name mb-0">@item.Baslik</h3>
                            <span class="cat-name">@item.İcerik</span>
                        </div>
                    </div>
                }

            </div>
        </div>
    </div>
</section>
@section Scripts {
    <script type="text/javascript">
        $(document).ready(function () {
           
        });
    function GetProje(id) {
         $.ajax({
                    type: 'POST',
                    url: '/Product/GetProje/',
                    dataType: 'json',
                    data: { id: id},
                    success: function (data) {              
                        // The json result will here
                        console.log(data)
                    },
                    failure: function (response) {

                    },
                    error: function (response) {

                    }
                });     
    }
    </script>
}

Controller:

public IActionResult Index()
        {
            var projes = _context.TProjects.AsQueryable().ToList();
            var cats = _context.Cats.AsQueryable().ToList();
            ViewBag.Proje = projes;
            ViewBag.Pcat = cats;
            return View();
        }

[HttpPost]
public JsonResult GetProje(int id)
        {
                var projes = _context.TProjects.Where(x => x.CatId == id).AsQueryable().ToList();

                ViewBag.Proje = projes;
                return Json(projes);
        }

Best Regards ,

Lewis