Office 365 keeps prompting for login credentials on Mac OSX? Office 365 keeps prompting for login credentials within Mac OSX? I came across a problem where a client of mine had bought and installed Office Suite 2016 she than upgraded to Office 365 Business Premium.
Here is what solved it for me. (Copied from: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_mac/authentication-failedcheck-your-account/063826c1-1e4c-40ba-9bfa-54e745925fc1 )


Quit all Office applications (Word, Teams, Outlook, etc)
Go to KeyChain Access.
Search "Exchange" under Login --> All Items and delete everything.
Search "Office" and delete everything.
Search "ADAL" and delete everything.
Launch Outlook.
You will get the activation prompt and then If account is already added you will see the password prompt for app and ADAL again. Please do 2-Factor Authentication if asked to.
If that doesn't work, do steps 2-4 but restart the Mac before launching Outlook.