We can look into the sales form template, Enviro-safe 28.

After you have set up your VAT registration number, it will automatically show on your sales transactions. Just click the Print or Preview menu at the bottom of the Invoice and you'll see the VAT Registration No. at the business information section. You can look into the screenshot below as your reference.

On the other hand, if you're referring to the VAT of an item so it'll automatically appear on the VAT column once you select it, you can set up a fix VAT rate of the item. Let me show you how:

1. Go to the Sales menu and select Product and Services.
2. Look for the item and click Edit.
3. Under the sales information, check the Inclusive of tax box.
4. From the Tax drop-down, select the tax type.
5. Click Save and close.

You can check out these articles that will help you in managing your VAT:

• VAT: Getting Started
• Set up and edit VAT settings, VAT codes, and VAT rates

I'm just around the corner if you need more help. You take care and have a great day!