Hi there, b-morrison90.
This can happen if the Form number on your invoice template is not enabled. You'll want to modify the template by following these steps:
Please see these sample screenshots for a visual guide:
Once done, go back and log in to your QuickBooks app and preview the invoice again.
I'll also include this article for future reference: Customize invoices, estimates, and sales receipts in QuickBooks Online.
If there's anything else you need, please let me know and I'll get back to you the soonest. Have a good day!