Warning: call_user_func_array() expects parameter 1 to be a valid callback

The warning is clear:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘cookie_GA’ not found or invalid function name in C:\wamp64\www\wordpress\wp-includes\class-wp-hook.php on line 286

So cookie_GA() is not found. When can this be the case? Well, of course when is_allowed_cookie('_ga') is false. So either wrap the add_action() call inside that if as well or create a dummy function.

if (is_allowed_cookie('_ga')) {
    add_action( 'init', 'cookie_GA' );
    function cookie_GA() {  /* your code */ }
}