Orcs must die- can't add to my game library :(

Huh, that's a bit odd. I've occasionally had an error trying to add a game, but not like that. Welp, wait and hour and try again, if it does the same, contact Steam support.

Wait, what...That seems extremely odd, How did you get that game?

I'm getting this error...

It's new Steam shield against Private Gifts

Well, you'll be glad to know that you're not the only one with this problem. The probable cause of this is that it unlocks bonus levels in OMD2, and as such, is being considered a DLC. The steam forums have a similar post, with numerous people affected on it, so it is very likely that they are aware of this. Hopefully, it should be fixed pretty soon for you, Hope it all works out!