Google Products Won't Publish - requiring MPN for custom; requiring optional fields

I sell customized shirts, bookmarks, keychains, etc. In my Google product fields I select “custom”. I keep getting the error saying they can’t publish to Google because either a GTIN/MPN is required or the size/gender/color is required. Has anyone resolved these errors? I’ve tried putting “0” in for the MPN just to give it something. I fill out the size/gender/color fields even though they are optional and it still gives the error.

I don’t know what it wants me to do anymore.