Use the following steps for login issues with Office 365 (either with Outlook 2016 for Mac or registering Office 2016 for Mac with Office365).
1. Make sure you 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