WebStore, Error Activating Windows 10 when using Parallels on Mac
When installing Windows on a Mac through the software Parallels, you may encounter an error with activating Windows. The error will report that "the product key was already used on another device." It looks like the following:
What to do when encountering an error activating windows after installing Windows through Parallels for Mac.
You can resolve this error by clicking the "Activate by phone" button and activating your copy of Windows through the phone. This is currently the only known way of resolving this error.