Not working starvoid key.

Steamgifts is a scam, etc . etc..

restart steam and then enter it again

Have you been attempting to redeem a few other keys today, such as from bundle keys, that were duplicates? If so, your account may have ended up getting locked out from redeeming keys temporarily. Don't be alarmed, it won't have used up your key, just wait for an hour or two and try again and it should work perfectly fine.