Why wont my twitter username change?

[Brit]

I've been trying to change my twitter username forever but its not working and its making me so mad. i made my twitter in like 5th grade and the usernames been @77angeluv since then and i hate it, but whenever i try to change it this comes up #<Alert:0x5dd46b8> why cant i change it? the username I want to change it to isn't taken so idk why its not working.

[Ghof]

try to change your username on another browser or PC
if they can not change too, delete your account and create again