What does this error mean?
This error is a direct message from eBay, stating that either
the return policy for the listing is missing,
or the policy does not meet certain criteria for the listing's category.
How to fix:
1) Missing Policy
If you're using a new eBay account, or haven't created eBay Business Policies yet, you will need to set up a Return Policy on eBay to publish listings with 3Dsellers.
See this quick guide: on how to set eBay Shipping and Return settings on 3Dsellers.
2) Category Requirements
eBay commonly updates rules on what return policy settings are allowed for certain categories and items.