Request content is invalid. Unsupported carrier was specified in line number 1. Only pre-defined names are allowed for 'carrier.'

Walmart returns this type of error in case the shipping carrier you specified is not one of Walmart approved carriers.

According to Walmart documentation, there is a list of approved carriers:

UPS, USPS, FedEx, Airborne, OnTrac, DHL Ecommerce - US, DHL, LS, UDS, UPSMI, FDX, PILOT, ESTES, SAIA, FDS Express, Seko Worldwide, HIT Delivery, FEDEXSP, RL Carriers, Metropolitan Warehouse & Delivery, China Post, YunExpress, Yellow Freight Sys, AIT Worldwide Logistics, Chukou1, Sendle, Landmark Global, Sunyou, Yanwen, 4PX, GLS, OSM Worldwide, FIRST MILE, AM Trucking, CEVA, India Post, SF Express, CNE, TForce Freight, AxleHire, LSO, Royal Mail, ABF Freight System, WanB, Roadrunner Freight, Meyer Distribution, AAA Cooper, Canada Post, Southeastern Freight Lines, Japan Post, Correos de Mexico, XPO Logistics, JD Logistics, YDH, JCEX, Flyt, Deutsche Post, Better Trucks, Asendia, SFC, UBI, ePost Global, YF Logistics, RXO, Estes Express, Shypmax, WIN.IT America, PITT OHIO, PostNord Sweden, Equick, Whistl, Tusou, Shiprocket, DTDC, PTS.

If you want to ship the orders with a different carrier, please set it as an Other carrier in your M2E Pro. You can find this setting under Walmart integration > Configuration > Accounts > Invoices & Shipments > Other Carrier. 

To add a shipping carrier, enter its title and website URL into the corresponding boxes. This way, your customer can follow their purchase on the carrier`s website by tracking the number included in the order. 

Once you configure all the options, please save the account settings.

This configuration will help to avoid this error for future orders.

You can use the Resend the shipping information option to remove the error for orders that already got it.