What are the minimum requirements for a custom Drupal 8 theme that can be enabled
Perhaps you've missed core.
Required params in *.info.yml are:
name
description
type
core
regions:content  

Also you can check your *.info.yml for using tabs instead of spaces:


  Tabs are NOT allowed, use spaces ONLY.
  You MUST indent your properties
  and lists with 1 or more spaces.


See more in official docs: https://www.drupal.org/theme-guide/8/defining-a-theme