Have searched and looked at other posts reporting similar issues? If it's the Xfinity My Account app you're trying to use and it goes to a blank screen when you try to make a payment, then there has been an update. There also is steps that may need to be done to get the update working properly. See https://forums.xfinity.com/t5/Xfinity-My-Account-App/Bill-pay-not-working-on-app/m-p/3243056#M2317
Welcome to the Xfinity Forums and thanks for posting to make us aware of the issues you're having when trying to make your bill. What is the make and model of your device? There was a recent update to the app. If you haven't already, can you please:
Once installed, please try to make the payment again and let me know if you have the same problem. If you do have the same problem, I'll need to begin troubleshooting.