Hi,
If you face any issues while installing the "Classy add-on for Elementor".
Then don't worry this is though a required plugin to install you can still import the demo data without this plugin.
But if you want you can collect the plugin from this link https://downloads.wordpress.org/plugin/classy-addons-for-elementor.zip
We will let you know as soon as the issue is fixed.
Thank You
SmartDataSoft Team