The add-on page here:
https://expressionengine.com/add-ons/cron
When you download from that add-on page the cron folder only has the pi.cron.php file. It's missing the addon.setup.php file.
When you download "latest" from here:
https://github.com/EllisLab/Cron/releases/tag/1.2.0
Same issue.
You have to download the mast repository to get the setup file.
FYI.