The plugin does not have a valid header.

When I try to activate the plugin I get the error “The plugin does not have a valid header.”

I see the other support thread for this same issue and a comment that it can be installed in the child theme header. I have no idea how to go about that, but more importantly, why I should have to do that. As a WP plugin, it should just be installable. Can you get this fixed?

Thanks.