You miss index.php as it is required – https://developer.wordpress.org/themes/getting-started/what-is-a-theme/#required-files
In style.css you have setup your theme as a child, that is second option for this message.
Parent theme is two file style.css and index.php. This parent theme style need to be without line about template – remove 7 line from this example if you have – https://developer.wordpress.org/themes/advanced-topics/child-themes/#2-create-a-stylesheet-style-css
Template line suggest using it as child theme rather than parent