Can‘t give a username for my Riot Account

Hi everyone! I have a little question… uhh I created a riot account a year ago or so and now I wanna give my account a username so that I can log in into my account. Sooo here is the problem: I tried sooo many usernames (also random combinations of numbers) and nothing works… I‘m so desperate. It says “Your Username is already taken by someone in another region”… please help me.