When you click the "download server" button in the launcher, for the most recent version of Minecraft (version 1.13), it causes your browser to go to https://s3.amazonaws.com/Minecraft.Download/versions/1.13/minecraft_server.1.13.jar to attempt to download the server. But there's a problem. The server jar is not located there. This may be more of a bug with the way the Mojang's AmazonAWS website is configured than the launcher itself, because the 1.13 server SHOULD be there, but it's not.