For the time being, I've been able to find a work around.
When I go to do an email merge to HTML and I get the error message, I go back and change the font and paragraph spacing. So far, changing both of these settings a few different times has fooled the system and it lets me conduct an HTML mail merge.
However, the software is very temperamental and you have to keep changing the font/spacing until it works. For me, I write my documents in Arial, and that triggers and error message when I go to merge. However, changing it to Calibri (body) has resolved
the issue. If it still doesn't work for you, I recommend adjusting your paragraph spacing settings, testing different configurations until it works.
Also, you have place a merge field somewhere in the email. (I think this is ridiculous because the PC version doesn't have this requirement). I just merge a blank field, because I do not personalize my outgoing emails.