A network administrator has a separate user account with rights to the domain administrator group. However, they cannot remember the password to this account and are not able to login to the server wh

Single sign-on is a technology that allows one set of credentials to access multiple systems - it does not describe a login failure.

Authorization determines what an authenticated user is permitted to do; the issue here is that the user cannot authenticate in the first place.

Access control is the broader framework encompassing both authentication and authorization, but it does not specifically describe the failure to supply correct credentials.

Authentication is the process of proving identity to a system, typically by supplying something you know (password), have (token), or are (biometric). When the administrator cannot remember their password, they are unable to complete the authentication step and therefore cannot gain access. The inability to present valid credentials is by definition an authentication failure, not a problem with authorization or access control policy.