Another backup folder issue

[Bulmer]

Whenever I try to backup anything (files, database or complete) I get the following error:
Symfony\Component\Debug\Exception\FatalThrowableError: Parse error: syntax error, unexpected '?' in /octobercms/public/plugins/panakour/backup/vendor/spatie/laravel-backup/src/BackupDestination/BackupDestination.php:138
Any idea why? I chose Local for destination.