I just did a fresh install of OSMC on RPi3 using the latest image.
Then I wanted to install Cron Task Scheduler via command line: sudo apt-get update
but when I ran: sudo apt-get install cron-app-osmc
I got this: unable to locate package cron-app-osmc
Thanks @sam_nazarko - I suspected that the package was missing as I was able to install smb package in the same way.
Just gave it another try and it worked perfectly - cron-app-osmc installed successfully.