Try copying the settings below:
Make sure you have the latest version of the client installed.
Try buying a package that is worth the remaining balance.
Make sure you have the latest version of the client downloaded for the F5 refresh feature to work.
Reboot your modem and see if that works. This fatal error is the result of connectivity problems.
Check to see everything is valid before hitting submit. Make sure you choose your country, as this is a requirement for all payments. If the problem persists, try filling out this form.
Try refreshing the page or logging in through a different server.
Another player asked about the same problem in the forum, and they were told to reboot their modem. If this doesn't work, then you should refer to this article for help.
I am going out on a limb here and saying it may be your keyboard that's driving you nuts. If this isn't an in-game bug, then you have to consider all troubleshooting options before reporting this to the developers. This article covers one plausible issue you may be having. If you know for a fact that your keyboard is up-to-date, then it may be a connectivity issue. In which case, I would suggest reading this article.
This guide mentions the inability to log out of an account once you log in or sign up on Steam. Developers are working to tackle this problem. Visit this topic if you have any further concerns.
Customer support service by UserEcho