I recently ran into this issue and decided to put a blog post together that walks you through the solution by creating a feed rule in your Google Merchant Center account. If you'd like, you can find out how to fix thesub-attribute [country (sub-attribute of [shipping])] issue here:
Hey there, I know I'm coming in way late on this but I may have found an easy solution. I had 38 products not approved because of the missing country sub attribute. Implementing the following fix immediately made them approved. Just go to the YouTube video link below and this guys walks you through the simple fix. He shows you how to easily add a rule that sets the country as US if it is not submitted by your Shopify content API.