The package could not be installed. The style.css stylesheet doesn’t contain a valid theme header.
What do you guys think went wrong?
Your theme’s stylesheet must contain a commented-out header with the theme’s name:
/*Theme Name: nameoftheme*/
I think the problem is you're trying to upload the entire zip file which also includes documentation and license information. You need to extract the zip to find the theme zip inside. Please check again.
First, theme support should be via the theme's WP Support page.
But it seems to me that the theme author didn't set up things correctly (hence the 'required license' problem). Which perhaps means that you didn't get the theme via the WP Theme Repository. Which might be not a good thing to do.
Themes require a certain structure and 'header' info. Themes that are submitted to (and approved by) the WP Theme Repository will be checked for required elements.
IMHO, risky to install themes from a non-WP Repository source. But, you should be asking via the theme support page, not here.