If you encounter errors when trying to buy keys, Steam Support recommends checking your or ensuring your account has no market restrictions (often caused by a new login or a recent password change).
To buy a key in-game at the standard $2.49 price, you generally must already own the corresponding case in your inventory. 4. Troubleshooting Purchase Issues buy csgo case keys
In late 2019, Valve updated key mechanics so that any key purchased in-game is permanently tied to the account that bought it. It cannot be sold on the market or traded to other players. If you encounter errors when trying to buy