You're in the right spot if you've seen an error from Walmart, for a SKU that you're trying to list that mentions: Unable to complete the product data setup process. Unable to override sku because there exists another source product from the same seller/org id.
Walmart only allows one SKU in the Seller Center per UPC so if you already have a SKU in your Walmart Seller Center for a given UPC then you want to remove that SKU in order to list another SKU with the same UPC.