I need to add a vat rate of 21% on invoices

I'll be happy to provide the steps you need, Silvia7.

You can use the Tax rate option by following these steps:

1. Go to Taxes on the left panel.
2. Click the Add tax button located in the upper right-hand corner.
3. Select Tax rate.
4. Enter the Tax name and Description.
5. From the Tax agency drop-down, select VAT.
6. Put a check mark on the Sales or Purchase box, then add the Rate.
7. Click Save once done.

Please see the sample screenshots below for a visual guide:

For more details, you can check out this article: Create custom tax rate. You can also visit our general support page to look for other guides and articles. 

Please let me know if you have other questions for QuickBooks. I'm just a few clicks away to help you some more. Take care and stay safe.