If you are having problems with Magento 2.3 about indexing and scheduled tasks in Plesk Onyx Panel, you have to reach the shell over SSH and run this command below:

/opt/plesk/php/7.2/bin/php /var/www/vhosts/yourdomain.com/httpdocs/bin/magento indexer:reindex

The important thing is here, you have to type full path of your php binary. If you directly run this command by starting with php command, your default php binary starts to work and you may get errors.