Hi, I have the same rejection:
"We found that your in-app purchase products exhibited one or more bugs when reviewed on iPad running iOS 14.7 on Wi-Fi.
Specifically, we found that when selecting purchases, then the buy **** in-app purchase button, an error message is displayed."
I have the same 'next steps' given, and I am also not doing server side verification. The error message that they mention in my case is an expected exception message when the App Store doesn't respond quick enough to the product request, so it's not actually bug.
Have you come to any conclusions?