I am having a similar issue only, I do have UPC barcodes that Google cannot seem to recognize for GTIN. I'm getting the error saying GTIN needed for a few of my products, but they all have UPC barcodes in the correct spots. Not having these issues with my many other products, so this is strange. Walmart is recognizing these UPC codes, just not Google.
I believe policy violation is something else. Either google doesn't like your image, your title or something in the description (or the actual product) They have a ton of restrictions of what you can and cannot advertise and their algorithms are over sensitive and sometimes you will scratch your head to try and figure out what they think is bad.
you have to actually go to google to find out what the violation is not within shopify. I only find it after clicking a bunch of things each time so I can't directly tell you how to find it I'm sorry. It's all so convoluted and confusing. It really should all be in 1 place to take care of everything, but it's not.
sometimes you can ask them to have a human review but I've not found that to work much.
PS: I edited that post. I solved my problem from below. Before you add that Google Sheet you must activate the mainfeed he asks. There should be some checkboxes before you can finaly start the sub feed.
Hey, we have the same issue with that mess. I did your instructions step by step and have now everything. But after i fetch and didnt see anything happens. I only have an error. It said "main feed not defined". Do you know what the problem is?