(Tagging @Kontur as well since he had the original question).
Follow the updated instructions on that page (the 4.17 should still be valid). If you are using anything newer than 4.14 - you don’t need to override GetSupportedPlatforms.
Your Build.cs should look exactly like this
using UnrealBuildTool;
using System.Collections.Generic;
[SupportedPlatforms(UnrealPlatformClass.Server)] // This line replaced the need for override GetSupportedPlatforms.
public class MyGameServerTarget : TargetRules
{
public MyGameServerTarget(TargetInfo Target) : base(Target)
{
Type = TargetType.Server;
ExtraModuleNames.Add("MyGame");
}
}