You configure how you sign in to the PowerPoint Add-in when you install the add-in. 

You can sign in to the add-in with your Anaplan credentials or Single Sign-on (SSO)

The add-in signs you out after 25 minutes of inactivity, or when you close PowerPoint.

If the failed login message displays, your credentials are not recognized. Make sure you've entered these correctly. If your credentials are correct, make sure your computer is connected to the internet. If you're still unable to log in, contact Anaplan Support.