I recognize how important it is to flawlessly change the sub-client billing option, CyberSpyder. I'm here to help you sort this out.

This error prompts if the transactions are under your sub-client while the payments are recorded under the parent. You'll have to ensure that there are no linked transactions on both sides before changing the type.

Here's how to change the customer's name on the sales transaction:

1. Select Sales on the left menu and go to the All Sales tab.
2. Open the transaction you want to update.
3. Change the customer name and click Save and close.
4. Click Yes to confirm the action.

Once done, you can now change the option to Bill this customer/client. Here's how:

1. From the Sales menu, choose the Customers tab.
2. Choose the sub-client and then click the Edit button.
3. On the Is sub-customer section, select the Bill this customer/client option.
4. Once done, click the Save button.

Additionally, I've included an article that'll help you personalize your sales forms in QuickBooks Online. This helps you add the info that matters most to your business: Customize Invoices, Estimates, and Sales Receipts.

I'm only a post away if you need more help in managing your sales transactions, CyberSpyder.  It's always my pleasure to help you out again.