Plugin service and variable
There's a lot going on here, but a) assuming you're really using Craft 3 and b) you generated your plugin, service and variable scaffolding on https://pluginfactory.io, this should work:

KGVariable.php Variable

&lt;?php

namespace kg\variables;

class KGVariable
{
    public function sayHello()
    {
        return \kg\KG::$plugin-&gt;useTR-&gt;exampleService();
    }
}


UseTR.php Service

&lt;?php

namespace kg\services;

use craft\base\Component;

class UseTR extends Component
{
    public function exampleService()
    {
        $result = 'hello';
        return $result;
    }
}


Then using this in a template will return hello:

{{ craft.kG.sayHello() }}