Call to undefined function insert_with_markers

Looks like insert_with_markers() function becomes available during admin_init hook. So in order for your code to work, you should do the following:

function do_my_htaccess_stuff_371705() {
//function `insert_with_markers()` is working now
}

add_action('admin_init', 'do_my_htaccess_stuff_371705');