Unable to select psaadm/root as system user in Tools & Settings >Scheduled Tasks.
Enhancing Security policy is enabled on the server: Enhancing Security.
According to Plesk documentation:
CONFIG_TEXT: To prevent users from running cron tasks and viewing the list of tasks scheduled on behalf of the root user, create an empty file named root.crontab.lock in the $PRODUCT_ROOT_D/var/ directory.
To set rot/psaadm users visible, rename file /usr/local/psa/var/root.crontab.lock.