What does "An unexpected error has occurred" mean on Steam when activating a key?

This generic error usually means a temporary Steam server issue prevented the activation from completing. However, this error is known to appear even when the activation actually succeeded — always check your Steam library and store.steampowered.com/account/licenses/ first before assuming the key failed. If the game is genuinely not there, check Steam server status at steamstat.us, restart Steam, try activating via the web, and make sure no VPN is active.