Hello there! 
I’m not here to send a solution but to give caution.
That prompt, “Email forwarding is not allowed for this domain,” led me to buy another domain since the first one I bought was a “.store.” I thought the problem might be because it is a “.store.” domain so I could not add a forwarding Email address. Because of that, I ended up buying a “.com” domain, and turns out that the same prompt still pops out.
So, my caution for everyone is not to try to fix it yourself because it costs me some money. But then the good thing is that those domains last for a year.
Hope this message helps and hope as well that Shopify will be able to fix the problem. 