.module file not being discovered
Just clearing cache is not enough, you need to uninstall and then re-install your module in order for the .module file to be discovered. 

In the Admin panel go to Extend and select the Uninstall tab. Select your custom module and click the Uninstall button. Now go back to the List tab and re-enable your module as before. Your implementation of hook_entity_insert() should now be recognised.