Can't understand why I'm receiving an error with codecept

I'm writing a small test program where I want to reach login form and to fill it. However, I receive an error.

Here's what I got in my code:

<?php

class loginCest
{
    public function frontpageWorks(AcceptanceTester $I)
    {

        $I->wantTo("Create login test");
        $I->amOnPage('my link');
        $I->see("join");
        $I->fillField('.form-control', 'my_login');
    }
}