Before you installing themes or plugins ?
WordPress gives errors such as plugin API error, Missing style.css if plugin or theme folder exists. Install easy theme plugin upgrade.
Log in to your WP admin area and go to Plugins > Add New.
Click browse, Point .zip file ex:- theme-essentials.zip
If you are installing plugin from WordPress.org,
Browse to the plugin archive and select it. Then click Install Now and the plugin will be installed shortly.