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
);
}