Dear annathaways,
Good day! Thank you for posting in Microsoft Community.
From description, you are getting the pop-up message “Excel is uploading your most recent changes. This will take a moment. If you'd rather not wait you can close the file now and we'll upload them the next time you open it.”
Recently, many customers reported similar issue, I'm afraid that this may be an issue caused by server side which needs to be investigated and fixed by the related team.
In this situation, we recommend you report this issue to the related team as they have higher permission to investigate this issue from back end.
Please contact your Microsoft 365 administrator and follow steps in this article to raise a support ticket.
For reference: How do I find my Microsoft 365 admin?
Thanks for your cooperation and understanding!
Hope you have a nice day and stay safe!
Sincerely,