Update 18 September 2020: Good news as some listings are seeing posts reinstated, so it probably means Google fixed the issue.
Some old Google My Business Listings didn’t get the Product Editor Feature when this was released last year. Unfortunately, it’s a well-known issue within the Google My Business community and Google has been struggling to fix.
However, they are rumours of the fix being implemented as I write this article.
But…as often with Google…something else breaks at the same time… this time appears that those old listings started to get posts rejected in bulk as soon they get the Product Editor feature enabled.
Hopefully, they will fix the Product Editor and Posts issue soon.