Can’t create child themes. “Stylesheet is not readable.”

The template name needs to match exactly the directory name of the parent theme, and you need a functions.php file in the child theme folder.

IE:

/*
 Theme Name:   Twenty Fifteen Child
 Template:     twentyfifteen

*/

…or just use this plugin: https://wordpress.org/plugins/child-theme-configurator/

and look at the child theme it creates to learn how to do it.