You need to make sure you have set up your BPay API credentials and entered them against the Merchant Facility. Follow the procedure: How to setup BPAY API Credentials
When setting up the credentials, please pay particular attention to the BPay Key/Token that is used and make sure that is the correct one for your Environment (Production/Sandbox). See Pre-production vs. Production.
You may also want to check the Environment on the Merchant Facility is correctly set to "Production" when in production mode or "Sandbox" when testing. For BPay, you'll need to have been approved by BPay for their production details and have the correct credentials from them.
If you might also pay to check you have the BPay Remote Site settings as active. Navigate to setup (cog in the top right), then search "Remote Site Settings". The edit and make active the following:
- BPayPreProduction
- BPayProduction
Finally, also check if your Payments2Us version is 9.4 and above if you are getting this error when using the checkout option to generate BPAY.