Server's (usually) don't have a Steam Client available to them. They are generally ran on server OS that doesn't have a monitor, doesn't have a Steam client and doesn't have a logged in user. As a result you need the Steam API to initialize as Steam Game Server, something Heathen's tool does for you with all Server Builds. You also need the Steam API to skip the launched from Steam Client check e.g. you need to have the steam_appid.txt located in the working folder of your server build, and populated with your app ID.