mysqldump must be available as it runs when you do it.
Its probably not in a path that is known to the web server user or whatever user your cron runs as.
You can find the path and then add it to the config file
https://spatie.be/docs/laravel-backup/v9/installation-and-setup#content-dumping-the-database
I prefer to put an env() helper here and defer to the .env file for the path.