Received response with code Error from Authorize.net: E00027: The transaction was unsuccessful.

I will add, according to this

https://developer.authorize.net/api/reference/responseCodes.html#search=...

"This error may occur for merchants on the HSBC or FDI Australia processors when setting validationMode to liveMode as these processors do not support authorization reversals. We recommend HSBC and FDI Australia merchants set validationMode to testMode instead."

But client has confirmed they are not on these processors.

Another note to this, authorize.net is sending a test transaction email every time an order is sent and fails. But, it is set to Live mode at Settings > General Security Settings > Test Mode, and the Drupal payment settings are also set to Live.