You typed the key. It says "Invalid." Don't panic.
If you are determined to get this specific pack, follow this priority guide: arma 2 reinforcements cd key
This happens if you try to install the "Standalone" version of Reinforcements without the base ARMA 2 data. Fix: You must own ARMA 2: Combined Operations (Base + OA) first. Reinforcements is just textures and scripts; it needs the core game to run. You typed the key
Some Arma 2 keys were region-locked to Russia, Brazil, or Asia. You can try activating via a VPN (against Steam’s terms of service but often works), or you must buy a global key. arma 2 reinforcements cd key
If you are coming from games like Call of Duty or Battlefield, Arma 2 will be a shock to the system.