Solution: resubmit the product to Walmart.
This error is due to the Walmart system having in its database information that your listing is supposed to end at one point. To correct this issue, you need to resubmit your item to Walmart with the Offer End Date set to a future date (e.g., year 2040)
You can update it through the GeekSeller General tab during product edition, or in bulk through CSV or XLS file.
You can also do that via the Walmart Seller Center by editing a product and ensuring the Site Start and End Dates are empty, then saving the changes.