Welcome and thanks for posting in the Community, @shannonkstone.
You're already on the right track in setting up or adding payroll to your client's company file. However, since the system is trying to charge them instead of adding them to your payroll subscription, I highly suggest contacting our Phone Support team.
They're able to pull your account in a secure environment using their screen-sharing (remote access) tool and check what's causing this behavior. They can also perform some troubleshooting steps if necessary.
To reach them, refer to this article to get their most updated contact information: How to contact QuickBooks Online Accountant support.
Just in case, I'll add this article for future reference: How to add payroll to a client's company file.
Please let me know in the comment if you have any other issues or concerns, and I'll get back to you as quick as possible. I'm always here to help. Have a great weekend!