There looks to be a typo in the Advanced segmentation tutorial.
https://lightship.dev/docs/moderate_semantictextures_tutorial.html
should the name be “sky” lowercase. When I changed this it worked.
    private void OnSemanticsBufferUpdated(ContextAwarenessStreamUpdatedArgs<ISemanticBuffer> args)
    {
        //get the current buffer
        ISemanticBuffer semanticBuffer = args.Sender.AwarenessBuffer;
        //get the index for sky
        int channel = semanticBuffer.GetChannelIndex("Sky");
        semanticBuffer.CreateOrUpdateTextureARGB32(
                ref _semanticTexture, channel
            );
    }