I am working on login with React JS and wordpress with the plugin JWT Authentication for WP REST API. I would like to get token but I have a problem

The code is here:

//...
function Login() {

    const[username, setUsername] = useState("")
    
    const[password, setPassword] = useState("")

    const access = async () => {
        
        const response = await fetch(`http://wp-playground.test/wp-json/jwt-auth/v1/token`, {
            method: 'POST', 
            header: {
                'Content-Type' : 'application/json',
            },
            body : JSON.stringify({
                username: username,
                password: password
            })
        })
        const data = await response.json()
        console.log(data)
        setUsername("")
        setPassword("")
    }

    return (
        <div>
            <h2>Login</h2>
            <label>Username: </label><input value={username} onChange={(e) => setUsername(e.target.value)}/><br/><br/>
            <label>Password: </label><input type="password" value={password} onChange={(e) => setPassword(e.target.value)}/><br/><br/>
            <button onClick={access}>Login</button>
        </div>
    )

and I get like this:

{
    code: "[jwt_auth] empty_username"
    data: {
        status: 403
    }
    message: "<strong>Error</strong>: The username field is empty."
}

Thanks for your attention!