Service not working - showing undefined function service()

I am tying to use service in Common.php like my other projects. But do not know why this is showing me the following error.

Fatal error: Uncaught Error: Call to undefined function service() in C:\Users\...\app\Common.php:19

Codes from Config/Services.php

PHP Code:

<?php namespace Config;

use CodeIgniter\Config\Services as CoreServices;

require_once SYSTEMPATH . 'Config/Services.php';

class Services extends CoreServices
{
 public static function testService( $getShared = FALSE )
 {
 if(!$getShared) return new \App\Models\TestModel;
 return static::getSharedInstance('testService');
 }
} 


Codes from App/Common.php

PHP Code:

<?php

if(!function_exists('test')){
    function test(){
        return service('testService')->test();    // Line number 19
    }
}

echo test(); 


Could you please help me to solve this issue? Thanks in advance.

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