I just bought the game, I try to start the game in the Java version and no matter what username I enter, it says it already exists.  EVEN IF I PRESS RANDOM KEYS!!!

I have spent more than 1 hour trying all kinds of key combinations and it simply does not accept any, I tried to log in from my microsft account in the browser and change my username directly from there, but no matter what I put it tells me that : The name already exists or it is offensive.

I'm a new player, HOW AM I SUPPOSED TO PLAY THE GAME I JUST PURCHASED IF I CAN'T START?