What if you cannot attend the citizenship test?
For whatever reason you need to let the immigration department staff know you cannot make the test by the date time proposed in the Email (You will receive an email if you are invited to the Citizenship Test).
Fortunately the immigration department provides with an online tool so that you can reschedule the date time by yourself.
https://appointments.homeaffairs.gov.au/login
Client ID is the the 11-digits number in the email body.