Anyone know why the MAX doesn't work?
What happened? It works fine at me and the "Max" sending button should be there.
Just go to the "Pay" button and click and you will choose either the fee rates in Mempool, Static, or ETA. And if you want to custom the fee setting just click "Advance". Everything works smoothly for me, the interface you've said will work fine too. Anyway, what OS you've used?
Otherwise try to check your internet connection and also the node of your wallet should be color green that indicates actively running.