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.
For example, sometimes, you must provide free 30-day returns for some items and categories. For specific details on what your policy must be, you may find the answer here in this article, or contact eBay support to get specific details.