Cannot create username

Upon purchasing game and instaling launcher i choose to play Java Edition and I have been asked to choose username. I chose one which was available (green check on right visible and no information below that this name is not available) and cliked "Submit". I have received information in polish which translates to " Whoops! Unexpected error...blablabla...try restarting launcher...or submit ticket". Whoops restarting launcher did not help. Whoops reinstaling launcher did not help. Whoops untinstallation of launcher, restart PC did not help. Whoops whats next with the game I bought and cannot play without Java Edition username.