Enforce use of 2FA in .net core application using identity

One ways is during login you check whether user have set the 2FA by :

var user = await _userManager.FindByEmailAsync(Input.Email);
var twoFactorEnabled = user.TwoFactorEnabled;

If it is false , you can then redirect user to config the 2FA page(./Manage/TwoFactorAuthentication) , after user set the 2FA and enable 2FA , TwoFactorEnabled in AspNetUsers table will be True and then during the login process , identity will automatically redirect user to ./LoginWith2fa page for 2FA login if current user's TwoFactorEnabled value is true .