That sounds like some steam issue. I mean it's not saying Duplicate Product Code, or giving the Invalid Product Code. Google had this to say

" What causes the “An unexpected error has occurred” error message on the Steam store? Some kind of a temporary processing issue caused by a hiccup on the Steam client's end or the server's end, or a problem with the particulars of your order such as the billing address you specified. An IP proxy or VPN being in effect."

I did check the key though and it matches w/ my humble key library. Can you give it another try perhaps later or after logging in and out of steam?