For allowing Students/parent to pay their payment online then you must have to enable any one of payment gateway, for this go to System Settings > Payment Methods then select any payment method from top of the page then enter your payment gateway account details in required fields then click on the Save button.

Once you saved the record you have to Enable payment gateway from the right side of the page and then click on  Save button.

Payment methods image

Note – You can add multiple payment gateways details but you can enable only one at a time.