Easypost returns this error when the carrier account you are using is not set as "Print As You Go".
Manifests are required for Print As You Go customers. They are not required for non-Print As You Go customers.
Here's the difference. A non-Print As You Go customer pays for the postage on purchase (each shipment is manifested separately). Non-Print As You Go customers are low volume (1 or 2 a day) customers. Print As You Go customers' shipments are not automatically manifested. Print As You Go customers have to manifest their shipments end of day.
To toggle "Print As You Go" you can log into your account and navigate to our Carriers Account Page, find your Australia Post account and click edit. There you should see a check box for "Print As You Go" which you can check or uncheck as needed.