The MerchantIdentifier provided in the XML payload is not valid for your credentials.
This error occurs when the MerchantIdentifier value you provide in your XML header is not valid for the credentials you provided in your HTTP header. Your credentials are the same as your user name and password for accessing Seller Central. The MerchantIdentifer value is your merchant token and looks something like M_ACMEINC_012345. This value is generated by Amazon when you register a new merchant account. To resolve this error, you must correct the MerchantIdentifier value in your XML header or modify your client settings to pass valid credentials in your HTTP header. If you do not know your merchant token, please contact us.