The ABN number will only show once you preview or print your invoice in QuickBooks Online (QBO). This is the reason why it isn't showing on the invoice page. You can view it by opening the invoice and selecting Print or Preview. Then, click Print or Preview again to view its details.
However, just make sure that the ABN checkbox is marked as checked in the invoice template that you're using. This way, it will display in the invoice header once you preview or print it. Let me show you how.
Open the invoice and click Customise.
Choose Edit current, then go to the Content section.
Click the upper body of the invoice and check the ABN box.