The sandbox and production environments are completely separate.
You cannot use your sandbox account API Login ID and Transaction Key on the production server, nor can you use your production account’s credentials with the sandbox.
If you do, you will receive an Error. Always make sure that if you are posting to the sandbox (https://test.authorize.net/gateway/transact.dll) that you are using your sandbox account’s API Login ID and Transaction Key. If you are posting to the production server (https://secure.authorize.net/gateway/transact.dll) then you must use your production account’s credentials.