Things you can do to fix this:
- Go buy the game and get a real CD key.
- Go on G2A and buy a key for really cheap.
- Borrow a copy from a friend and give it back, you now have a CD key.
- Buy the game and sell it back, you now have a CD key.
- You loaned the game to a friend and now he has your CD key.
- Someone stole your CD key.
- That free CD key you found online is in use.
- Try and run the iw3mp.exe as administrator.