STEAM VERIFICATION
Sign in to the Steam account
Use the login and password the store sent, with "Remember me" checked.
Wait for the code screen
Keep the next Steam screen open. Do not close it!
Redeem your code here
Now click Redeem code and copy your Steam verification code. Use it within 5 minutes. If an error occurs, sign in to Steam again and try the same code within that time.
SINGLE USE CODE
You can paste it with or without dashes, upper or lower case.
Check that you pasted the whole code, exactly as you got it.
MOST LIKELY CAUSE
The right order is: sign in to the Steam account first, wait for the screen asking for the verification code, and only then redeem here. If you redeemed before that, the code was already spent.
Unable to access? Contact the seller and request your code manually.