Update doesnt work

Hi this is my code.. I just wonder if why I still encounter error in Category.Update even if the model is mapped all fields with Repository. The Category.Add works..

protected DbContext _context { get; set; }
public SupplierService(DbContext context){
_context = context;
}
public void Create(CategoryModel model){
_context.Categorys.Add(model.CovertToRepository());
_context.SaveChanges();
}
public void Edit(CategoryModel model){
_context.Category.Update(model.ConvertToRepository());
_context.SaveChanges();
}