Hey there, paul113,
Let's get this sorted out so you can lodge the client's Single Touch Payroll (STP).
You may have registered the supplier ABN, and that's why the setup failed. It is the business ABN that must be registered. The tax agent is NOT permitted to contact the ATO on behalf of the business. Therefore, an authorised contact of the business must contact the ATO to register.
I've got the steps to enable electronic lodgement so you can use STP:
Electronic Lodgement & STP
Update details via Access Manager
You can also refer to this article: How do I enable Single Touch Payroll? for more details about the steps provided above.
If you're getting the same results, I recommend contacting our QuickBooks Online Payroll Team. They can do screen-sharing privately to ensure you have a successful STP setup.
Let me know if you have follow-up questions. Just mention my name, and I'll get back to you.
Thank you and stay safe!