Theme error: Template missing?

This is the message I get on my themes page for this site.

“ERROR: Template is missing. Standalone themes need to have a index.php template file. Child themes need to have a Template header in the style.css stylesheet.”

Not sure I understand. The theme (Nisarg), which I recently installed, seems to be operating as expected.

The index.php is in the theme folder. Don’t think I touched it.

I did one thing wrong, I know, which was to copy the whole style.css file into a child theme. I’m a beginner, though I have been a member for a while.

Any suggestions appreciated.