castopod/app/Language/en/Validation.php

<?php

/**
 * @copyright  2020 Podlibre
 * @license    https://www.gnu.org/licenses/agpl-3.0.en.html AGPL3
 * @link       https://castopod.org/
 */

return [
    'not_in_protected_slugs' =>
        'The {field} field conflicts with one of the gateway routes (admin, auth or install).',
];