Amazon Return Code Error for Shipping

I purchased something from Amazon and intended to return it because I didn't like it. I applied for the return to MNG Cargo using the code provided by Amazon. MNG Cargo, on the other hand, informed me that a return code beginning with D1 was incompatible with their system. As a result, MNG Cargo refused to accept my return using this code. I'm having difficulty returning the shipment and am unable to contact Amazon for further assistance. I'm having trouble because the code I received from Amazon isn't recognized by MNG Cargo. This problem is preventing me from completing the return process, and Amazon's lack of communication is making matters worse.