I don’t mean offense, but your searching skills seem to be hurting you. You don’t want to only search microsoft’s site for a windows question, that would depend on microsoft’s own search function to be good & for them to have an official and easy to find answer.
When searching google for windows “challenge phrase” I saw many results. Instructions on how to deal with it.
Now if it specifically says “enter the A1B2C3” that’s different. It seems to be a particular bug, I saw procedures that were supposed to fix that. You need to search for windows “challenge phrase” "A1B2C3”. Search link
I don’t use windows myself, so my familiarity isn’t the best. I was wrong to jump to talking about reinstalling windows. If you have data or information stored on the computer that you don’t want to lose, then reinstalling windows probably should be a last resort. If you have no data you care about, then it could be the easiest option for you.