Right-click the game’s executable (not the launcher). Select Properties > Compatibility.
This is the #1 culprit. Sniper Ghost Warrior exists in two parallel universes: sniper ghost warrior serial key is invalid
The Fix: If you bought a key from a third-party site and it says "invalid" inside the game, try opening Steam and activating the product there instead. If that works, you have a Steam key, not a retail key. Conversely, if you are trying to put a retail DVD key into Steam, it will always fail. Right-click the game’s executable (not the launcher)
If you bought a Steam key from a third-party marketplace (G2A, Kinguin, etc.), you might have received a key for a different region or a key that was originally part of a physical bundle (e.g., a GPU promo). Conversely, if you own a physical disc from 2010 and are trying to activate it on Steam, it will not work—retail disc keys from that era are separate from digital distribution platforms unless explicitly stated. The Fix: If you bought a key from