I'll share some information that could help you.

You'll want to check your QuickBooks Online subscription. If you are on Simple Start or Essentials, the Customs Form Styles on the settings are unavailable. The feature is only available if you're on the Plus version.

You may want to consider upgrading your QBO version to utilize this feature. You can visit this page for more information and see the other feature available on different versions: https://quickbooks.intuit.com/sg/pricing/.

You can also utilise this article for future reference: Customise invoices, estimates, and sales receipts in QuickBooks Online.

For additional QuickBooks-related concerns, don't hesitate to post them here in the Community. We're always available and willing to lend a hand to your queries. Have a great day ahead.