Symfony \ Component \ HttpKernel \ Exception \ MethodNotAllowedHttpException. What is the cause?

I'm quite new to Laravel and I'm facing a vague error. Whenever I try to login with an username and password, I get this error.

Symfony \ Component \ HttpKernel \ Exception \ MethodNotAllowedHttpException

My code consists of this:

UserController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Auth;
use App\User;

class UserController extends Controller
{
    public function postSignUp(Request $request){
        $firstName = $request['firstName'];
        $lastName = $request['lastName'];
        $username = $request['username'];
        $password = bcrypt($request['password']);
        $email = $request['email'];

        $user = new User();
        $user->first_name = $firstName;
        $user->last_name = $lastName;
        $user->username = $username;
        $user->password = $password;
        $user->email = $email;

        $user->save();

        Auth::login($user);

        return redirect()->back();
    }

    public function postSignIn(Request $request){
        $username = $request['username'];
        $password = $request['password'];
        if (Auth::attempt(['username' => $username, 'password' => $password])){
            return redirect()->back();
        }
    }
}

Provider called User.php

<?php

namespace App;

use Illuminate\Contracts\Auth\Authenticatable;
use Illuminate\Database\Eloquent\Model;

class User extends Model implements Authenticatable
{
    use \Illuminate\Auth\Authenticatable;
}

Route file web.php

<?php

Route::get('/', 'PagesController@index')->name('home');
Route::post('signup', 'UserController@postSignUp')->name('signup');
Route::get('signin', 'UserController@postSignin')->name('signin');