Contact Technical Support if you see this message.
This issue can happen if you have previously tried to redeem or use your activation code on a fake McAfee website.
There are many fake websites on the internet that claim to offer McAfee Technical Support and services. If you visited one of these websites by mistake and typed in your activation code, the cybercriminals running that website might have obtained it. When you next try to use the activation code, the Key already in use message indicates that the cybercriminals have already used your activation code.
Contact McAfee Technical Support using the link above if you think your activation code might have been used already. For more information, see TS102485 – What to do if you think you have used a fraudulent McAfee Technical Support website.