Can't Assign Role to User

I tried to use spatie(laravel-permission) on my laravel apps but I can't assign role to the user. The error I facing is

Call to undefined method Illuminate\Database\Query\Builder::assignRole()

AdminsContoller.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use App\Http\Requests;
use App\Http\Controllers\Controller;
use App\User;
use Spatie\Permission\Models\Role;
use Spatie\Permission\Models\Permission;
use Spatie\Permission\Traits\HasRoles;
class AdminsController extends Controller
{
    /**
     * Display a listing of the resource.
     *
     * @return \Illuminate\Http\Response
     */
    public function index()
    {
        $users = User::all();

        return view('admin.dashboard', compact('users'));
    }

    /**
     * Show the form for creating a new resource.
     *
     * @return \Illuminate\Http\Response
     */
    public function create()
    {
        //
    }

    /**
     * Store a newly created resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\Response
     */
    public function store(Request $request)
    {
        //
    }

    /**
     * Display the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function show($id)
    {
        $user = User::whereId($id)->firstOrFail();
        // $user->hasAllRoles(Role::all());
        return view('admin.show', compact('user'));
    }

    public function assignRole($id){
        $user = User::whereId($id)->firstOrFail()->assignRole('Staff');
        redirect('/dashboard');

    }

    /**
     * Show the form for editing the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function edit($id)
    {
        //
    }

    /**
     * Update the specified resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function update(Request $request, $id)
    {
        //
    }

    /**
     * Remove the specified resource from storage.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function destroy($id)
    {
         $user = User::whereId($id)->firstOrFail();
         $user->destroy();

         redirect('/dashboard');
    }
}

via Chebli Mohamed