Sophia here from Shopify. If you're seeing this error message in the Walmart Marketplace app, please get in touch with the app developer so they can investigate with you. You can email them at WMA_Support@walmart.com. Alternatively, you can select "Get support" on their Shopify App Store page.
I see that you posted this question nearly one month ago, so if you've already come across a solution, please feel free to share it here. Thanks so much!