I'm having the same issue with my parallels 11 license. I recently migrated to a new mac, but I made sure to sign out on my old computer. When I attempt to activate the license I get the "Failed to activate the license: Unable to confirm the product license. You have reached the maximum number of activations available for your activation key (http://www.parallels.com/eula (http://www.parallels.com/eula)). Please purchase a new product license." error. I know I'm not signed in on any other machines. This happens both in the app and from the terminal with prlsrvctl activate-license-online.