That error is coming from a macro, which is probably in an add-in. To diagnose the cause, you need to disable or uninstall each add-in, one at a time, until you find the one whose removal makes the messages stop appearing.
Go to File > Options > Add-Ins to see what's loaded. Make a note of the Location value for each active add-in that's described as "Template" in the Type column. (You probably need to test only the "template" type of add-in, because the other types don't usually
cause macro-type errors.) Then close Word.
Now go to the location of one of the templates and move its file to a different folder (such as C:\temp) where Word won't find it during startup. Restart Word and repeat the situation that usually causes the error message. If the message still appears, shut
down Word, restore the add-in you moved, and move out a different one. Eventually you should find one that makes the message stop. Then you can go to wherever you got that add-in and ask to have it fixed.