Error Role Administrator does not exist in identity mvc

Question

hi everyone,

 

i registered name email n contact number in identity ... then i put coded in controller here below

public async Task Register(RegisterViewModel model)
{
if (ModelState.IsValid)
{
var user = new ApplicationUser { Name = model.Name, UserName = model.Email, Email = model.Email, IsActive = true };
var result = await UserManager.CreateAsync(user, model.Password);
if (result.Succeeded)
{
// error shown like this below in red color
UserManager.AddToRole(user.Id,RoleName.AdministratorRoleName);
// RoleName.AdministratorRoleName ="Admin" given value
UserManager.AddClaim(user.Id,new Claim(ClaimTypes.GivenName, model.Name));
await SignInManager.SignInAsync(user, isPersistent:false, rememberBrowser:false);

Anybody can solve? please support me