Inside your hook_menu
implementation, try to get rid of %
at the end of the path definition because it is not needed.
function HOOK_menu(){
$menu = array(
'get_tax/autocomplete' => array( // THE EDITED LINE.
'page callback' => 'tax_autocomplete_callback',
'page arguments' => array(2),
'type' => MENU_CALLBACK,
),
);
return $menu;
}
If it still does not work, kindly copy & paste the error message you see.