The Countries report shows your performance broken down by the country of the visitors who have viewed the ads on your site. You can view a country breakdown when viewing reports by product, custom channel, URL channel, owned site, platform, ad unit, ad size, ad type, bid type, and targeting type.
The country breakdown of reports by URL channel, custom channel, ad unit, and ad size is only available for reports with date ranges starting on March 9, 2013, or later.
The Countries report is not available when viewing reports by site. For insights into your performance on a site level, use the Owned sites report