You'll get this error message if you have other sub-customer that has a zero open balance. To isolate this, make sure that they have a balance, and they're information set to Bill this customer not Bill with parent.
1. Go back to the Sales menu, then Customers tab. 2. Click the name of the sub-customer. 3. Select the Edit option at the upper right.
4. Under the Is sub-customer box, choose Bill this customer on the drop-down. 5. Hit Save.
Once done, you can now create a statement for a sub-customer.
Aside from that, you can also manage, edit, customize, and print statements in QuickBooks Online (QBO). Check out this article for more information: Create and manage statements.
Please let me know if you have any other issues or concern, and I'll back to you right away. Take care and have a great one.