Occasionally Microsoft Office will loose its mind and repeatedly ask for you to sign in even though you know you're using the correct credentials. You'll see a screen similar to the one below over and over again.
To fix this, first quit Outlook (and any other Office application like Word, Excel or Powerpoint).
Use Spotlight to search for and launch the Keychain Access application. You can manually launch it from the Applications >> Utilities folder.
With Keychain Access open:
Select "all items" at the bottom left.
From the search box at the top-right, enter "adal" (minus the quotes).
Highlight all the entries containing MicrosoftOffice15_2_Data:ADAL and delete them
From the search box at the top-right, enter "office" (minus the quotes) and delete any items containing "Microsoft Office Identities Cache."
Finally, launch Outlook or another Microsoft Office application. You'll be prompted to login to authenticate Office. Do so and you should be back in action!