Not able to play singleplayer

Greetings,

I was gifted the game by a friend of mine through a key and have been unable to play singleplayer. The message "Requires purchase of base game" appears.

I've googled a bit and think it may be possible for me to be on a free client? Is there a way to double check that it is indeed the free client and not just a bug? I'd rather not buy the DLC thing if it's unecessary.

It's left me a bit confused and I'd appriciate any help.

Thanks.