Could you please locate to the root directory of the Office installation, such as the 64-bit version, C:\Program Files\Microsoft Office\Office16, find the executable programs for Word (Winword.exe) and Outlook(Outlook.exe).
Right-click on them > Propertites > Details, check the Verion number of Word app and Outlook app.
On my test environment, I could reproduce this issue.
Besides, is there any third party software or updates installed before this issue occurred?
Here is a similar thread, you could have a look, "Mail merge to HTML format - not doing anything.". In addition to the marked answers on the first page, there is also a sharing on the second page about Skype for Business.
Any updates, welcome to post back.
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