The error message "You are not authorized to set up customized products for this category" indicates that UPC Exemption is being used for this product, but a UPC Exemption has not yet been granted by Walmart for the related listing.
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To remedy this, you can either contact Walmart Support and request that exemption be granted for the SKU, or alternatively you can remove the UPC Exemption value and supply a valid UPC/GTIN.