Unity Ads is an ad network for iOS (iTunes) and Android (Google Play). Unity Ads does not currently support the following:
Web platforms (e.g. WebPlayer, WebGL)
Amazon App Store, Amazon Marketplace for downloadable Android games
Note: You can implement Unity Ads in Android games downloaded through Amazon, but Unity campaigns currently advertise apps for the Google Play store. Therefore, ad impressions will not generate revenue unless the user also has access to the Google Play store.