Let me share additional clarifications to your questions to continue your work, @CyberSpyder.

Thank you for following my peer's response. The linked transactions that my colleague @CharleneMaeF is referring to are the delayed charges from the customer and sub-customer that you linked to an invoice. This made the error appear when you try to change from Bill with parent to Billing with customer.

To get rid of the error message, you have two options. You can remove the sub-customer charges in the invoice or delete the invoice itself.

If you choose to remove the charges, ensure to take note of the charge amounts and the dates so you can remove them by hand. You'll see this in the sub-customer Transactions List. The value may be slightly different due to tax.

Here's how to remove the sub-customer charges in the customer invoice:

1. Go to the Sales menu and then select the Customers tab.
2. Choose the customer's name and then click the Transaction List.
3. There you'll see the invoices. Pick the invoice that may contain the sub-customer charge.
4. Above the Customer label, click the [no.] linked transactions.
5. Select Remove.
   
6. Hit Save and close.

If you choose to delete the invoice transaction and recreate it after switching sub-customer to Bill with client, you can follow these steps:

1. In the customer Transaction List, select the invoice.
   
2. Click More.
3. Choose to Delete.
   

This will leave the charges open when you recreate the invoice. You can now continue to change the sub-client from Billing with parent to Billing with client/Billing this customer.

On the other hand, as you can see in my screenshots, the name Chris there is the parent, and the sub-client is Sample. The sales transactions that you should change would be under Chris' name. For more details about using the parent billing option on your invoices, you can read this article: Use the bill with parent option on invoices in QuickBooks Online.

However, if the error still appears, I'd recommend contacting our Care Support team to investigate this further. They have tools to pull up your account and see the situation you have in a private session.

Would you have additional questions regarding the parent billing option or any product-related concerns, please tag me in your comment. I'm willing to assist you through this forum. Have a nice day ahead.