Use the (add) link present there to add the VAT tax.
Once setup VAT will either be automatically added or subtracted from your invoice total depending on the setting you chose.
Edit tax settings on an individual invoice by clicking (change settings) at the top of the invoice editor.
Adding VAT or tax numbers to clients invoices:
Go to Setup > Custom Fields.
Add a Invoice detail field called “VAT number” there and save.
Now enter the clients VAT number by going to the client record in SnapBill.
Click Modify at the top of the client details table and enter the VAT number in the “VAT number” field.
To get this field to display on invoices please ensure the field type is set to Invoice detail at Setup ⇒ Custom Fields.