It looks like the issue comes down to Word on Mac using a different setup than the Windows version, so the “File > Options” menu you saw online isn’t available here.
On macOS, this usually happens when the edit preferences aren’t set right or Word has a temporary glitch.
The easiest fix is to open Word, go to Word > Preferences > Edit, and make sure “Drag-and-drop text editing” is turned on.
While you’re there, uncheck “Smart paragraph selection” because that can mess with highlighting.
After that, restart Word and, if needed, your Mac to clear any stuck processes.
If it still doesn’t work, does this only happen in Word or do you notice the same problem in other apps too?
That will help figure out if it’s a Word setting or something system wide.
Regards,
Aron