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We may not be able to use Office Web Apps to mail merge. Office Web Apps is used to extend Microsoft Office experience to the web browser, where we can work with documents directly and more convenient.
While Office Web Apps are the light version of Microsoft Office, some features are not available in web browser.
In this situation, we can mail merge with local Office and upload the word to SharePoint Online Team site, and then print them online.
We also sincerely suggest you to submit feedback about your requirement with the following link, to and this would help us to improve our products and services.
Thanks for your understanding.
Is any more assistance needed here?
If so, please feel free to post here as we are very glad to help you.
Thanks for your time.