The amount of VAT added to your invoice should comply with your tax regime.
You can easily modify the amount of VAT when creating an invoice. First, open an invoice template by clicking on "Create invoice".
To modify VAT, you just need to click on the "VAT (%)" field and choose the correct % amount from the dropdown menu.
For an e-invoice which has no VAT applied, you should put "0" in the VAT field, and then choose the reason for VAT exemption, by clicking on "Select VAT type". There are seven different reasons on that list which correspond with the codification of the Italian Revenue Agency. This system will not allow you to save an e-invoice without selecting the relevant reason.
In any scenario where you are unsure about the amount of VAT you should apply, or the reason for the VAT exemption, we would always suggest for you to consult with your accountant before proceeding.