"An unexpected error has occurred. Your purchase has not been completed."

First of all, sorry for asking for help on this subreddit.

Secondly, I've been trying to buy Bit.Trip Runner 2 all day, and everytime I try it fails, showing that

"An unexpected error has occurred. Your purchase has not been completed. Please contact Steam Support."

I already opened a ticket on Support, but something tells me they won't answer it soon. What can I do?

Cheers!