Steam showed an error when I activated my key but the game isn't in my library — what do I do?

First check store.steampowered.com/account/licenses/ — if the game is listed there, it was activated. Restart Steam and it should appear in your library. If it's not in your licenses either, the activation genuinely failed. Check Steam server status at steamstat.us for outages, then try activating again at store.steampowered.com/account/registerkey after restarting Steam and disabling any VPN.