I dealt with this frustrating issue for over a year, but here's how you can fix it.
Example image:
How to fix:
1. Make sure Finder is open and all office apps are closed, including Microsoft Word.
2. Click on "GO" in the top bar.
3. Once the drop down menu descends, hold down the OPTION key on your keyboard. You'll see that a folder called "LIBRARY" appears. Click on "LIBRARY".
4. Once inside "LIBRARY", click on a folder called "CONTAINERS."
5. Scroll down until you find a folder called "com.microsoft.Word".
6. Once you've found it, delete "com.microsoft.Word" off your computer.
7. Now, reopen Microsoft Word, and make sure the upload pending error message is gone. It should be, and you should now be able to delete the file if you so choose.
Also, if you're experiencing this problem with Excel and/or Powerpoint, you can probably fix it the same way. You'd just have to delete "com.microsoft.Excel" and/or "com.microsoft.Powerpoint" respectively, instead of "com.microsoft.Word". All the steps leading up to that would remain the same.
I really hope this helps other people who are dealing with this problem. And if for some reason it doesn't work for you, reach out to Microsoft support while signed into your Microsoft account and get into a chat room. Chatting with a specialist was the only reason I was able to figure this out in the first place.