If you see this error appearing on all or most of your new products being listed to eBay, be sure to make sure that your Account Default Return Policy in eBay complies with eBay's new Return Policies guidelines.
On June 1, 2018, eBay introduced a reworking of their allowed return policies. You can read more about the new guidelines from eBay here.
"Return Policy Attribute returnDescription Not Valid On This Site" means that you must adjust your return policies in eBay itself before you can list new products.
"You've provided an invalid return policy" also means that you must adjust your return policies in eBay itself.