Hello SJT2009,
After seeing your post I did a little test on Word for Mac 16.16.12 and unfortunately I could not reproduce the issue you met.
Please refer to this article to modify your spreadsheet format:
Prepare your Excel data source for a mail merge in Word for Mac
Also, it's suggested to use corresponding mail merge types to start, you can follow this article:
Use mail merge...
To narrow down the issue, can you please provide the following information for moving further:
- Does this error message appear when you open a specific Excel spreadsheet or all ??
- A screenshot of the issue could help us understand your environment much.
If it still can't solve the problem, you could share the file with me via PM(click here)for further assistance.
What's more, you can click Help->check for updates to keep Office apps updated.
Hope it could help.
Best Regards,
Oliver