Thank you for posting, @leonie3.
If you're referring to your VAT ID, that'll show once you've added your VAT Registration No. from your VAT settings. You can check that information by going to the VAT menu from the left tab. Click the Edit VAT button at the top and select Edit VAT settings.
However, if you're referring to the VAT on the invoice itself, make sure to add a VAT rate on the invoice that you've created. When creating an invoice make sure to select a rate from the VAT column.
You can run Transaction List by Date report to view all invoices that you've created without VAT.
To generate the report:
Check these article for more information:
Let me know if there's anything that I can help. I'm always here to assist.