Sometimes a key is incorrectly reported by Steam as already being used.
Is it just one key from your order? Or are multiple keys affected? Please try the remaining keys in your order and not if any others are causing problems.
Please try logging out and (after a pause) back into your Steam client and then try activating the key again.
If the problem persists then we can help you get a replacement key. If the order is less than 30 days old then please open a support ticket and let us know:
- Your order ID
- The key and product that doesn't work
We will check the key with Steam and replace if necessary.