Hello Alex,
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.