"Charge and Complete" is always greyed out. How do I enable it?

Any time you want to charge and complete a service the amount of the payment needs to match the amount due for the service. After selecting your payment type and entering the correct amount for the payment, you must them click off of the payment amount box so that the system will recognize the amount entered. After the system recognizes that amount you should be able to click the "Charge and Complete" button.