string errors = JsonConvert.SerializeObject(ModelState.Values
                        .SelectMany(state => state.Errors)
                        .Select(error => error.ErrorMessage));
                    return Json(new { url = "error", errors });

//Or
string errors = string.Join(',', ModelState.Values.Where(x => x.ValidationState == Microsoft.AspNetCore.Mvc.ModelBinding.ModelValidationState.Invalid).ToList());