Intel Parallels 17 Pro, but earlier version of Parallels (and non-pro) did this as well. I don't think it's a Para problem, pretty sure it's an MS problem. I had this same problem (below) with a Windows 10 Home key as well, which is ultimately why I rebought the new Windows 10 Pro key. Not long ago upgraded to P17 and created a brand new VM using a brand new Windows 10 Pro key. Had a couple unrelated iPhone syncing issues and decided to completely fresh format the iMac. Reinstalled and activated P17 no problem, but when I go back to create a new Windows 10 VM the brand new key that has only been used once before - and only for Parallels 17 Pro - isn't accepted. What do I do? I'm not buying YET ANOTHER Windows key (I'm up to 14 different total Windows keys I've purchased for my household). (sorry for the small image, can't get Parallels tools installed either because I can't get past the activation part)
