Unable to backup PST files in Outlook 2016 or 2019 on Windows 10

I have one user out of 80 who cannot get DLO to back up their .PST file.  The error is this:

The pst file C:\Users\<username>\Documents\Outlook Files\Outlook.pst was skipped because it is not configured in Outlook.

This is, of course, not true.  It is very much configured.  Otherwise the user wouldn't be able to use Outlook or access their .pst file.

Originally, Office 2019 was installed and gave us this error.  I removed 2019 and installed 2016.  Same error.  I tried it with a different .pst file (i.e. "outlook1.pst" instead of the default "outlook.pst").  Same error.

Nobody else has this problem.  Can someone help us out, please?  Thanks!