When Alocating an invoice number to invoice in app It is showing reference as correct number but on actual invoice/ preview it is not showing invoice number at all?

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:

1. Log in to your QuickBooks Online account using the regular browser.
2. Go to the Gear > Custom Form Styles.
3. Select the Edit link of the template you are using for your invoices.
4. Click the Content tab, and select the header part of the invoice. This will display the options on the right menu.
5. Put a checkmark on the Form numbers box.
6. Hit Done.

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!