Impossible to choose a username

Hello everyone!
I just bought minecraft, and when I want to launch the game from the launcher, I'm asked to choose a username first, except that no matter what I write, the same message "this username is already taken" appears. So I'm stuck...

Note that I had possibly already bought minecraft with this microsoft account, but after having tried everything to find the game without success I resigned myself to buy it again. Is it possible that the bug comes from there? 

thanks!

Translated with www.DeepL.com/Translator (free version)