Can't attach role on RegisterController

Hello,

I'm still a beginner at laravel. I'm trying to auto assign roles to new user in different controllers. In the EmployeeController :

public function store(StoreEmployeeRequest $request)
    {
        $employee = Employee::create($request->all());
        $user = User::create([
            'name' => $request->full_name,
            'first_name' => $request->first_name,
            'last_name' => $request->last_name,
            'email' => $request->professional_email,
        ]);
        $role = Role::where('title', 'Staff')->first();
        $user->roles()->attach($role);
		return redirect()->route('employees.index');
}

In the StudentController :

public function store(StoreStudentRequest $request)
    {
        $student = Student::create($request->all());
        $user = User::create([
            'name' => $request->full_name,
            'first_name' => $request->first_name,
            'last_name' => $request->last_name,
            'email' => $request->school_email,
        ]);
        $role = Role::where('title', 'Student')->first();
        $user->roles()->attach($role);
		return redirect()->route('students.index');
}

For both Employee and Student it's working, but now I'm trying so that any user that self registered will be registered as a Guardian.

I tried this code on the RegisterController :

protected function create(array $data)
    {
        $newUser = User::create([
            'first_name'     => $data['first_name'],
            'last_name'     => $data['last_name'],
            'email'    => $data['email'],
            'password' => Hash::make($data['password']),
        ]);
         $role = Role::where('title', 'Guardian')->first();
        $newUser->roles->attach($role); 
        return $newUser;
    }

But it's not working : Method Illuminate\Database\Eloquent\Collection::attach does not exist. What should I do? Please help.