Welcome aboard, @EAZZY.

Let me ensure that you’ll be able to add VAT numbers on your invoices in QuickBooks Online (QBO).

Beforehand, let’s turn on your VAT in the QBO company file to keep track of your state's tax laws and accurately calculate VAT and returns: Set up and use VAT in QuickBooks Online

To add the field when invoicing, we’ll have to utilize the Custom field in QBO. Take note that regardless if you’re using a free version of QuickBooks, you’ll still be able to perform the steps below. Here’s how:

1. Go to the Gear icon.
2. Under the Lists column, select the Custom field.
3. Click the Add field button. Then enter the VAT number under the name section.
4. Tick the box for All Sales forms, and toggle to turn on the Print on form. This way, the field will be added when creating any sales form and will be shown when printed out.
5. Then, press Save.

I also added some screenshots for your visual preference.

Once done, you can then create an invoice and manually enter your customer’s VAT number. You’ll want to click the Print or Preview button to double-check what your invoice will look like and ensure that the field has been added.

In addition, I included this article when customising your sales form in QuickBooks Online: Customise invoices, quotes, and sales receipts in QuickBooks Online

Let me know if you’re able to customise your sales form. I want to make sure that I have assisted you accordingly. Have a great day!