getting Unrecognized token '<" when json.stringify data send from react-native to laravel controller

I am trying to post data from react native app by json.stringify to Laravel controller. But getting "JSON Parse error: Unrecognized token '<'". I need help how to resolve it. My code are given below:

react-native end:

fetch(API_URL+'/signup', {
        method: 'post',
        header:{
            'Accept': 'application/json',
            'Content-type': 'application/json'
        },
        body:JSON.stringify({
            name: userName,
            email: userEmail,
            password: userPassword
        })

    })
    .then((response) => response.json())
        .then((response) =>{
            alert(response);
        })
        .catch((error)=>{
            console.error(error);
        });

Laravel:

public function registerUser() 
{

    $this->validate(request(), [
        'email' => 'required|email',
        'name' => 'required',
        'password' => 'required|min:4|confirmed'
    ]);

    $user = User::create(request(['email', 'name', 'password']));

    return "success";
}