I use Printful for some printed items to sell on my Shopify store for almost two years now.
I've always been able to edit the description in Shopify after it migrates from Printful. However, today I am not able to on any products from Printful. I can change the image, but if I alter anything in the description box it gives me the attached error message.
Most likely issue is that you've duplicated the product, and need to replace the product/variant images.
You assigned images to the product/variant that previously existed somewhere else in your store, and they've since been deleted, so now those images are gone and are causing the issue to show.
If you can't figure it out, based on the above, then just try making the product again from within Printful, publishing it to your store, and then you should be able to edit it without issue.
I use Printful for one of my stores, and I can confirm that I am not receiving this issue (just tested it), so it's probably not an issue with Printful, but rather your files/images being used (guessing you deleted something you shouldn't have, at some point).
Hope the above helps! 🙂
★ Did my post help? If yes, then please like and accept solution. ★