This question already has an answer here:
PHP: “Notice: Undefined variable”, “Notice: Undefined index”, and “Notice: Undefined offset”
You should first check if your element exists in your array:
if (array_key_exists('aktifko',$_COOKIE) AND $_COOKIE['aktifko'] != 'XXX')
With your current code, you are checking the value of this element in the cookie array. But if it's the first time for the user, this element won't exist in the array. So you have to check if the key already exists before checking its value.