I would suggest you try to use Excel online and Power Automate to Mail Merge.
For more, please checkout this video.
(Please Note: Since the web site is not hosted by Microsoft, the link may changed without notice. Microsoft does not guarantee the accuracy of this information.)
If you need the further help on Power Automate, It's recommend to post a new thread on Microsoft Power Automate Community, because these issues are not supported on Q&A forum ~
Thanks for your understanding.
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