Failing to download ESP8266-Board

Hi, I want to program an ESP8266 with Arduino, so I tried to download the ESP8266-Board, but when I click on "install" it says "Failed to download [link]". I tried it with multiple versions, but it didn´t work. Does anybody know how to fix this?

Where exactly did you get this message ? Have you added the URL for the ESP8266 to the Additional Boards Manager dialogue in the IDE ?