Ouch, that error message doesn't sound good. I'm assuming this came up from sprint.com. Have you tried using a different brower or computer. You can always dial *3 from your Sprint phone to pay your bill over the phone. Here are other options to pay the bill as well:
Let me know if you need any further assistance.
Sprint Social Care
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.