I appreciate you for performing the steps provided by my colleagues.

Could be that the reason you don't have this option is that you haven't set up your GST yet. If so you can check out this article for the process to help you set it up: Set up GST and BAS in QuickBooks Online. After successfully setting it up, you may follow the steps provided by my colleague above. These steps:

1. Click the Gear icon and choose Custom Form Styles.
2. Select Edit on the Master Invoice (or the frequently used invoice template).
3. Hit the Content tab and then click the top box of the preview page.
4. Underneath the Header, tick the box for the Australian business number (ABN).
5. Scroll down and tick the box beside Customer ABN under the Display sub-menu.
6. Click Done.

You can utilize this article as your future reference: Customise invoices, quotes, and sales receipts in QuickBooks Online.

Feel free to post here again if you have further QuickBooks-related concerns. We are available 24/7 to lend a hand with each of your queries. Stay safe!