The first thing to check is whether you’ve disabled the expansion packs using the Decks button in the New Game screen. It can be easy to forget!
On Steam, there is an issue with Steam itself where this can occur. Quitting and restarting Steam fixes the problem in most cases. You can also try these steps:
- Restart your computer.
- Right click the game in Steam, click Properties, click Local Files, then click Verify Integrity of Game Cache.
- If you are playing offline, use the “Go Offline” feature of Steam.
You may also try contacting Steam support.