I hope that I have correctly identified the issue in the title.
Specifically:
1. I have been doing mail merges using MS Word and MS Access for a very long time.
2. I migrated from Office 2003 to Office 365 many months ago and was able to continue doing the mail merges that I had been doing.
3. The last time I was able to do the merges was in August 2020.
4. I attempted to do a merge today without success.
5. When I try to perform a merge with any of the documents previously used documents I get a window labeled "Data Link Properties". This has never happened before and there is nothing apparent to me that I can edit that will fix the issue.
6. My preliminary conclusion is that an MS update has caused the problem.