How do I learn to tell what makes a piece of writing work?

Right now a piece either lands or it doesn't, and you can't say why. That is a normal place to start. Taste comes before the ability to explain taste. You already feel more than you can name, and the naming is a skill you build, not one you are born lacking. The way in is to slow down your own reactions. When something works, you usually feel it before you understand it. So catch the feeling first. Where did you lean in? Where did you drift? Those reactions are data. Editing is just learning to trace them back to the choices on the page that caused them. Pick one thing you admire and one that fell flat, and read them side by side. Ask what the good one does that the weak one doesn't. Openings, pace, clarity, honesty. Do this a few times a week and the fog lifts. You are not waiting for a gift. You are training an eye.