A strange thing happened, when I installed W10 with "MediaCreationToolx64 "a few days ago it did not ask for a key (the 6 figure Pin), & this time wants one & my bought DVD W8 key is not accepted?, anybody knows the reason?
I think you should install from Windows Update to upgrade your Windows 7/8/8.1. Everyone who has a genuine version of previus version of Windows can get a genuine Windows 10. So after the upgrade you can check your product key with a program called ProduKey so you can have it in case you need to make a clean install.
What do you mean by this time... have you installed it on a different machine or are you try to do a fresh install (instead of upgrade) over the first one?
Both senarios will now ask you for a key but the steps to fix it are different.