Some customers are exempt from paying sales tax. You need to acquire a tax-exempt number from the customers who tell you they are exempt. QuickBooks provides a place where you can enter this number. Then when you issue an invoice to that customer, tax is not charged. You don't have to make any changes on the invoice because the process of not taxing particular customers becomes automatic.
Double-click on Customer, not Job. When making changes such as indicating a tax-exempt customer, be sure you open the Customer's edit window, not the edit window for a Job. There is no option for setting sales tax information in the job windows.