Your first issue is happening with this code -
return pc.ValidateCredentials(username, password);
No credentials were found, returning a 'NULL' value.
The second part then, based on the NULL value is -
return RedirectToAction("Login", "Account");
This will return you back to your Login form. So, start looking at why no Credentials were returned and your issue will be resolved.
Richard is also spot on, use Windows Authentication -
C# using Windows authentication to login in to app[
^]