Hello there @Msb0216!
I’m sorry you are having a hard time scheduling a payment arrangement. I am elated to help!
The easiest way to set up a payment arrangement using your phone is to make one using the myAT&T app. No need to worry the process is very simple! All you need to do is log in, and tap menu. After tap Billing & payments then Make a payment. Complete the payment form, choose a Payment date, and select Payment Method. Lastly, hit continue; review the terms and conditions, and then Submit. You can also find more information on this process here! 
Thank you for letting me help you! Have a great rest of your evening.
Cesar, AT&T Community Specialist