Refer to the exhibit. Why is user authentication being rejected?

While username format mismatches can cause authentication issues, the primary reason for rejection when the TACACS+ server is down is its unavailability and the lack of a successful fallback.

TACACS+ servers support various authentication methods, including CHAP, so a user configured for CHAP would not be inherently refused unless there's a misconfiguration in the server or client, or the server is unreachable.

If the TACACS+ server is down but the user is present in the local database, local authentication should succeed, not be rejected.

If the primary TACACS+ server is down, the authentication process typically attempts to fall back to the router's local database. When the user's credentials are not present in this local database, or if local authentication is not configured as a fallback, the authentication request will be rejected.