I find the (insert any MUTU move) difficult/painful/pointless - can I substitute it or leave it out?

Ok so if it’s hard, then its challenging you – either in terms of proprioception, balance, core strength or simply fitness or muscles strength. So you should probably keep trying that one. 

If it hurts, stop doing it right away. Revisit the audio guidance and make sure you’re moving slowly, mindfully and according to the instructions. 

Pain is your body’s very clear signal to you to stop. So let’s say for example that your ‘knees hurt’ when you squat or lunge. These are movements that (notwithstanding any previous injury or medical condition that is stopping it) your body should be able to do. 

So don’t leave them out completely and read this article Squats, Lunges Technique and Modifications