If you have an installed Windows virtual machine that often prompts for the activation key, you might follow the suggestions below.
Does a Windows license come with a Parallels Desktop license?
A Parallels Desktop for Mac license does not include Microsoft® Windows. A license for Parallels Desktop for Mac is the license key that allows you to activate only Parallels Desktop. This license key isn't supposed to activate Windows. More information can be found in KB 9003.
To activate your Windows virtual machine, follow the steps outlined in article KB 114051.
Note: If you have a BootCamp-based virtual machine, refer to KB 9703.