With Microsoft 365 requiring Modern Authentication if you have Outlook 2013 it needs some Registry keys to be added to enable that in Outlook 2013. Outlook 2016 or newer do not need any registry changes since Modern Authentication is already enabled.
In notepad copy and paste the following into the file and save it as Outlook2013-ModernAuth.reg in a location where you can get to that file. Then on each user with Outlook 2013 double click this file and then choose Yes to allow it to add the registry entries.
Windows Registry Editor Version 5.00
To see the page at Microsoft about this – Enable Modern authentication for Office 2013 on Windows devices