Add a cron that runs the scheduler every minute:
* * * * * cd /path-to-your-project && php artisan schedule:run >> /dev/null 2>&1
In Plesk, the command will look something like:
cd /var/www/vhosts/domain/httpdocs/ && php artisan schedule:run >> /dev/null 2>&1
Replace "domain" with your real domain.
Make sure to set the "Run" to "Cron Style" and insert:
* * * * *