get_search_form() and aria_label

You can pass an array of attributes with aria_label attribute in it.
Try this:

get_search_form(array('aria_label' => 'search-form'));

This function gets a form from searchform.php by default and if you have this file in your theme and aria-label still missing, you need to check the code in this file. Sometimes developers do not include an option to set an aria-label.