Help me for code

API
2

Dont need this header

Don’t leak generated tokens

The prefix for helix is Bearer not oAuth so your headers should be:

return fetch(`https://api.twitch.tv/helix/users?login=${username}`, {
    method: "GET",
    headers: {
        'Client-ID': "csj7l3h1nqr7iyd1vudl79q21dcen9",
        "Authorization": "Bearer MyToken",
    }
})

Code example demonstrating this kind of call Twitch Implicit Auth Example