Dear Intern7,
Good day! Thank you for posting to Microsoft Community. I am happy to help you.
Based on my experience and feedback from other users, this phenomenon may be related to the version of Outlook.
You can first try to install the classic version of Outlook app can send via mail merge:
You can't open classic Outlook on a new Windows PC - Microsoft Support
If you are still experiencing the same problem after using this version, I would like to further confirm the following information so that I can continue to investigate the issue:
I look forward to your reply. Thanks for your cooperation.
Sincerely,
Tin | Microsoft Community Moderator