Thanks to a client that was experiencing this issue with Outlook on their MAC and adding another mailbox, continually got this message, “Can’t add Office 365 Sharepoint right now”
Found the answer here: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_install-mso_mac/cant-add-office-365-sharepoint-right-now/888b9c4a-4e5f-40c7-86d2-61daadbd5f82?tab=question&status=AllReplies
Try the below suggestion to see if the issue can be fixed:
1. Make sure you have quit Outlook and other Office apps. Go to KeyChain Access.
2. Search “Exchange” under Login –> All Items and delete everything.
3. Search “Office” and delete everything.
4. Search “ADAL” and delete everything.
5. Launch Outlook.
6. You will get the activation prompt. If the account is already added, you will see the password prompt for app and ADAL again. Please do 2-Factor Authentication if asked. And then you should be able to login.