Access Server: Google Authenticator multi-factor authentication

How to Enable Google Authenticator MFA for Access Server 

OpenVPN Access Server supports the Google Authenticator MFA system, but it is not enabled by default. To enable it globally:

1. Sign in to our Admin Web UI.
2. Click Authentication > General (Access Server version 2.7.5 and newer) or Client Settings (Access Server version 2.7.4 and older).
3. Enable Google Authenticator MFA, save and update your server.

Note: If you are using MFA added by post-auth script, enabling Google MFA will break user authentication. Ensure that no other MFA is enabled when enabling Google MFA.

See the full guide here: Google Authenticator multi-factor authentication

Access Server Resources:
OpenVPN Access Server Documentation
OpenVPN Access Server Resource Center
OpenVPN Access Server Admin Manual