Please be advised that eBay depreciated Return Policy Descriptions for some sites besides DE, ES, FR and IT.
Please find relevant eBay documentation here: https://developer.ebay.com/devzone/xml/docs/reference/ebay/ReviseItem.html#Request.Item.ReturnPolicy.Description
Please also note, that warning messages do not prevent listings from being created on eBay. In the message you can also find an Error message, which prevents it from being listed. Please fix the issue stated in the error message and try to list again.
I tried to list new products today with linnworks and I could not list them because the following error popped out "Warning (21920200): Returns policy attribute returnDescription not valid on this site Error parameters: returnDescription."