Raffle Glitched For Me.

you got it during free weekend try syncing your games by clicking acount>sync account then it might get rid of the free weekend sanctum

Resync your profile on Account->Sync Account in this site.

Currently there is an issue with sync system due to Steam Community problems, some of you have no Sanctum and synced already after free weekend period, but still unable to enter. Try to clear the cookies and cache, restart your browser then sync again, this may help. If it doesn't, wait a bit and try again.
Edit: of course if you haven't synced yet.
Edit: Also this thread is useful too.

If you installed the game during the free weekend, you may want to make sure you uninstalled it. Whenever a free weekend ends, valve just removes the listing for that game in your library on steam. The game is still installed on your computer, in your steam directory. You can remove it by going to Control Panel, then "Programs and Features" (Win7) or "Add and Remove Programs" (Win XP). See if the game is listed there and uninstall it. That was the only way I could find to remove it. Then I resynced my account and I can enter Sanctum giveaways.