You must first create the Tax. See How do I add/remove a Tax to/from a Service?
To view, add/remove a Tax to/from a Service, from the menu tap Manage > Services
Tap on the relevant Service
Tap the Taxes tab to VIEW ONLY any existing Taxes that have previously been applied to this Service:
To add / remove a Tax from this Service, tap EDIT
Scroll down the Service box to the Taxes section
Slide the enable button to the right next to the relevant Tax(es) you wish to apply to this Service: