freaking eBay removed compatibility option to some previously-available eBay Motors sub categories, no warning! - nothing to do with sixbit!
Resolution: Had to export all compatibility, and change Category Name and ID to one that is allowed by eBay. then re-Import (update)
I still don't know what is the need of having to select a category for these presets...meh.
I am also having this problem, but it is not an issue with eBay as it was for the above seller. I am selling in category 183721 on eBay Motors, and I am getting that same error "Parts compatibility is not enabled for this category." If I go list directly on eBay without going through Sixbit, I can put the fitment data on the item. It is an intermittent problem that happens on some items and not others within the same category. I have developed a workaround by changing the category completely and then changing it back, which usually works. I'm on 3.000.241