Why can't I select text in MS Word using wireless mouse on Mac mini?

MS Word supports non-contiguous text selection by double-clicking the first word, and then pressing the cmd-key and double-clicking the next word. The following from MS Word v16.80 on macOS Sonoma 14.2:

I believe you misunderstand the problem. "Something" happens that prevents the ability to select in the normal way - clicking and dragging. My experience is that it comes and goes, and may be linked to a particular file in Word. I also sometimes have the same problem in Outlook, where rebooting the application (rather than the computer) mostly clears the problem. However this does not work in Word, where the problem appears to be file related. Selecting is still possible by double clicking on a word, or triple clicking to select a paragraph.

I am sitting here in front of a document opened with MS Word v16.86 and I have no issue with what I posted previously, or a single click followed by a drag to select any text.

If the mouse you are using is battery-powered, or rechargeable, check that, and possibly power it off and re-pair it with the Mac. Otherwise, how a non-Apple mouse behaves with macOS and Word should be, but is not guaranteed to be predictable.

Using an Apple Magic Mouse II on macOS Sonoma 14.5.