This error would occur if you try to install Cornerstone separately on Pro.
Please note that if you are using Pro, Cornerstone is already native to it so it does not need to be installed as another plugin as that would trigger the error.
When you edit a page on Pro, instead of seeing Text/HTML/Cornerstone, you will see Text/HTML/Pro.
So kindly login through FTP then go to wp-content/plugins then delete the current cornerstone directory that is in there.
Hope this helps.