Purchase Error While Trying to Buy a Game

try to:
1) Logout from steam; reboot your PC; login back and try to purchase.
or 2) Start a game; Alt+Tab it; try to complete a purchase
I had the same error during Black Friday sale and found these tricks on steam forums. Alternatively this error will disappear in a day or so.

You might be better off finding someone in your friends list who can buy the game for you, and just repay them back in keys later on.

I think I got this message once (it might have just been a similar message). I was able to make a purchase through my web browser, which seemed to fix the problem with the client too, as I have purchased things since then.

I got this before, contact steam support and resist the urge to write an angry tirade in the support ticket. I missed three really good games on sale because of the error but they bundled them all later eventually so it worked out in my favor.

I have the same error, and a lot of peoples have this error and valve dont care about us, this is @#%@$#@