Cloning the Base theme bartik, seven, and stark in Drupal 8
A few things that could have been missed -

Cache clear,
Changing the name of the theme in .info.yml of the new theme,
Not putting the base theme in the .info.yml.

If none of those work, this is a pretty good guide. https://www.drupal.org/theme-guide/8/creating-a-sub-theme