[Return Location] When I create a return location, I get the following error: "Listed but failed to authenticate contract".

You have been "Declined" because the information on the return location contract is inaccurate.
You cannot use Goodsflow (automatic return pick-up service) for any product that is listed with this return location. 

If you cannot create a correct return location,
enter "returnCenterCode" : "NO_RETURN_CENTERCODE" when you create a product, and list your product without a return location code.