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.

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.