Why is user_can_access_admin_page() an undefined function?

As mentioned by Jacob Peattie: functions not loaded yet.

You will need to use hooks/actions (init, wp etc) https://codex.wordpress.org/Plugin_API/Action_Reference

e.g.

function callMyMethod(){
    $userLoggedIn = is_user_logged_in();
    echo 'INIT Action: User Logged in: '.var_export($userLoggedIn, true);

    if ( is_admin() ) {
       $canAccess = user_can_access_admin_page();
     }
}
add_action('init','callMyMethod');

This answer is assuming you’re creating a plugin and calling the function on the main/root page.

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Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)