Laravel post error

I'm getting the following error message when trying to update user data:

 protected function methodNotAllowed(array $others)
    {
    throw new MethodNotAllowedHttpException($others);
    }

I'm logging the user in, then want to give them the option to change their preferences. The form displays fine in the view but won't post.

Here are my routes:

Route::prefix('admin')->group(function(){
Route::get('/login', 'Auth\AdminLoginController@showLoginForm')->name('admin.login');
Route::post('/login', 'Auth\AdminLoginController@login')->name('admin.login.submit');
Route::get('/', 'AdminsController@index')->name('admin.dashboard');
Route::post('/', 'AdminsController@update')->name('admin.dashboard.update');
Route::get('/logout', 'Auth\AdminLoginController@logout')->name('admin.logout');

Here's the Controller:

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Admin;
use Auth;

class AdminsController extends Controller
{
/**
 * Create a new controller instance.
 *
 * @return void
 */
public function __construct()
{
    $this->middleware('auth:admin');
}

/**
 * Show the application dashboard.
 *
 * @return \Illuminate\Http\Response
 */
public function index()
{
    $id = Auth::user()->id;
    $admin = Admin::find($id);
    return view('admin')->with('admin',$admin);
}

public function update(Request $request, $id)
{
  $this-> validate($request, [
      'target_sector' => 'required|max:255',
      'target_skillsets' => 'required|max:255',
      'target_companies'=> 'required|max:255',
      'target_locations'=> 'required|max:255',

  ]);
  //Create Post
  $id = Auth::user()->id;
  $admin = Admin::find($id);
  $admin->target_sector = $request->input('target_sector');
  $admin->target_skillsets = $request->input('target_skillsets');
  $admin->target_companies = $request->input('target_companies');
  $admin->target_locations = $request->input('target_locations');

  $admin->save();

  return redirect('/admin')->with('success', 'Preferences Updated', 'admin',$admin);
}

}

And here is the view:

@include('includes.nav_login')
@extends('layouts.app')

@section('content')

<div class="container">
    <div class="row mt-4">
        <div class="col-md-10 offset-md-1">
            <div class="card">
                <div class="card-header">Admin Dashboard</div>

                <div class="card-body">
                    @if (session('status'))
                        <div class="alert alert-success">
                            
                        </div>
                    @endif

                    You are logged in as ADMIN!
                </div>
                <div class="card-header">Update Vacancy Preferences</div>
                <div class="card-body">
                   {!! Form::open(['action' => ['AdminsController@update', $admin], 'method' => 'POST']) !!}

                    <div class="form-group">
                      
                      
                    </div>

                    <div class="form-group">
                      
                      
                    </div>

                    <div class="form-group">
                      
                      
                    </div>

                    <div class="form-group">
                      
                      
                    </div>

                      
                      

                    {!! Form::close() !!}
                </div>
            </div>
        </div>
    </div>
</div>
@endsection

Can anyone explain why this isn't working?

via Chebli Mohamed