The "merchant_shipping_group_name" (Free Shipping) field contains an invalid value.
Within your SellerChamp default Amazon account settings page, under the Default Shipping Template option, an incorrect shipping template name has been entered.
Login to your Amazon seller central account and go into Settings. Within your settings page, go into your Shipping Settings page and under there find your Shipping Templates. Note the name of the template exactly how it is there and enter the same name of the template within SellerChamp.