This license key is already registered message

I have a license key for Parallels Desktop 11 which I have not used for a few years. Now that I have upgraded to a M2 MacBook Pro and have installed Parallels Desktop for Mac (not the App Store version), my license key will not activate the software. When I try registering the key in my Parallels account, I receive the message: This license key is already registered. What can I do to activate Parallels?