You're using Microsoft Outlook 2016 for Mac version 15.20 or later versions to connect to an Exchange Online mailbox through a federated account, and modern authentication is enabled.
When you try to connect to the Exchange Online mailbox, your provider’s Active Directory Federation Services (AD FS) authentication dialog box is displayed, and you enter your credentials. However, you're repeatedly prompted for your credentials after you've already entered them.
This issue occurs if the following conditions are true:
Your email address differs from your user principal name (UPN).
You entered your email address in the User name field of the Accounts dialog box in Outlook 2016 for Mac.
In this situation, you have to manually enter your UPN in the Username field in the AD FS dialog box of your provider.
Enter your UPN in the User name field of the Accounts dialog box, and then quit and restart Outlook 2016 for Mac.
If you don't know your UPN, contact your administrator.