This version is not compatible with Drupal 7.x and should be replaced.

I created a fresh sub-theme and deleted all the unwanted content and file's from my new sub-theme folder, On appearance page I was unable to enable it as it was showing me an error

this version is not compatible with Drupal 7.x and should be replaced.

This make me confuse as what I mistakenly deleted that this sub theme not working. Then What I found in my info file that I was missing two important parameter defined as below

These two could a reason if your subtheme also show the above mentioned error.

1. You might have forgot to define core = 7.x in .info file,
2. Or You might have forgot to define regions[content] = Content in .info file

Once you fixed it please refresh the page now hopefully you will be able to Enable the theme

Thanks