When I follow the steps for generating a sub-client statement, I get the following error. A sub-customer is being billed with its parent customer

Hi there, GGBAP Firm. I'll help you take a look at what's going on with creating your document.
 

I accessed our test drive account and created a sub-client, marking them as billed with parent, then tried repeating the same steps you're doing. Here's the message it shows:

The last part of this notification explains what to do. Since your client is billed with its parent customer, you'll have to create a statement for them instead:
1. In the left navigation bar, go to Sales, then Customers.2. Choose the parent customer for your sub-client.
3. Use our Batch actions drop-down menu and pick Create statements.
4. Enter the appropriate details.
5. Click Apply.
6. Select Save.

That's the solution to the error you're receiving. You'll now have a statement produced for your parent customer which will show details for their sub-client.
 

I've included a couple helpful resources below about working with sub-customers in QuickBooks Online which you may find useful:

• Manage your customer list
• What does the bill with parent feature let you do on invoices?

Feel welcome to send a reply if there's any other questions. I'll be here to assist. Enjoy the rest of your day!