سلام وقت بخیر ، این کد رو توی DbContext قرار بدین

  protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            foreach (var relationship in modelBuilder.Model.GetEntityTypes().SelectMany(s => s.GetForeignKeys()))
            {
                relationship.DeleteBehavior = DeleteBehavior.Restrict;
            }
            base.OnModelCreating(modelBuilder);
        }