I've tried this in my company, and I'm here to share the steps with you, Izzy8.
The VAT Registration No. won't show up on the invoice if you haven't set up the VAT feature.
Let's set it up by following these steps:
- On the left panel, click VAT and then select Set up VAT.
- From the Set up VAT page, choose the Start of VAT period, Filing frequency, Reporting method, and your VAT Registration No..
- Click Next, and then select Got it.
That should display the VAT Registration No. when you print your customer's invoice.
For reference, here's an article on how to set up sales GST/VAT rates and use them on forms? (International QBO).
Please update us with the result. You can also reply on this post if you have other questions. Have a wonderful day ahead!