When you equip Chargebee with the API Login ID and Transaction Key unique to your account, Authorize.Net recognizes it as a trusted application and surrenders transaction information to it. 

This error pops up when the Login ID and Transaction Key you have qualified Chargebee with to connect and interact with Authorize.Net are no longer valid. To fix this, you must update these credentials at Settings >>Configure Chargebee>> Payment Gateway >> Authorize.Net >> Configure this gateway  on Chargebee. 

If you don’t remember your Transaction Key, you can generate a new one at Account >> Settings >> Login ID and Transaction Key on the Authorize.Net interface.  

Warning: On occasion, Authorize.Net disables old API keys even when the Disable Old Transaction Key(s) option is left unchecked (see screenshot). To avoid errors, always use your new transaction key after it is generated.