Thank you for contacting Steam Support.

Are you attempting to make the purchase through Steam or with a web browser using the storefront webpage?

If using Steam, try the following:

If using a PC
-Exit Steam, then go to the location that Steam is installed to. By default this is C:\Program Files\Steam directory (this is the default with a Steam installation - if you set a different installation directory, you will need to browse to it).
-Delete all of the files except for:
Steam.exe
And the folder \SteamApps\

If using a Mac:
-Exit Steam and go to the folder Users/[username]/Library/Application Support/Steam
-Delete all of the files in this folder except for SteamApps.

Restart your computer and test the issue.

Note: This process will not affect your currently installed games.

If the issue still persists, try using a web browser (Internet Explorer 7 or higher on PC, or Safari if on a Mac) to make the purchase using http://store.steampowered.com/

If it still fails, try logging into your Steam account and making the purchase directly through the Store interface.

Please let us know if you have further questions or concerns.