When you set up your item, you put an offer end date that occurred in the past. Listing Mirror does not receive or pass an "end date" data field. This error has appeared on Listing Mirror accounts when the listing has already been created outside of Listing Mirror.
To resolve this issue, check to see if any other required fields are missing data within the edit listing page. Checking to make sure the variation name and variation value are filled out (if applicable) and that category is selected. Also, check category specific fields that might be required as well. Please note that all required fields will be marked with a red asterisk.
You can also try to adjust it via Walmart Seller Center (seller.walmart.com) by editing a product:
One last item to note: For ALL Walmart listings, a UPC or GTIN will be REQUIRED to list to Walmart.com