ATO Machine Credentials have a life of two years. Upon expiry, APS Tax AU will throw an error message when the expired credential is attempted to be used.
If the machine credentials have expired, you will get the error Test has failed: Exception [STMException], Message = Expired Initiating Party Certificate during the pre-lodgement and lodgement of the returns.
Resolving the STMException error
To resolve this error:
Obtain new credentials from the ATO.
Once you have obtained the machine credential, follow one of these articles to set them up in APS Tax Manager.