I was under the impression that you were loading the data into a object that you created, but you ar laoding into a objcy from the entity model.

try this

public ActionResult CreateBulkCommissions()
        {
            var month = DateTime.Now.ToString("MMMM");
 
           
 
                List<TutorCommission> TutorComs = db.Enrollments.GroupBy(g => new { g.Tutor ).Select(s => new TutorCommission
                {
                     CommissionAmount = s.Sum(u => u.CommissionAmount) * s.Key.TutorCommissionPercentage,
                     TutorNoID = s.Key.TutorNoID ,
   CommissionMonth = month,  // string constant 
                      CommissionStatus = "Unpaid"
 
                }).ToList();
 
foreach (var newCommission in TutorComs
            {

                db.TutorCommissions.Add(newCommission);
                db.SaveChanges();
  
 }
            return RedirectToAction("Index");
        }