In order to resolve the error 13801 "IKE authentication credentials are unacceptable", please follow these suggestions:
Firstly, make sure that you are not connected to any other VPN service, as this may cause interference when connecting to an Atlas VPN server.
Additionally, after you receive the error message, you can try to connect again, as sometimes, the error is a one-time occurrence.
If the issue still persists, open Windows Search, look up VPN Settings, and open it.
In there, find AtlasVPN IKEv2 Connection, and click on it. This will expand additional options. After this, click on Remove and try to connect to the Atlas VPN servers.
If you are still receiving the error message, please try reinstalling the Atlas VPN application.