Why can't I build a diet plan that actually works?

A diet sheet is a set of rules. A plan that works is a fit between a person and their real life. Those are different skills, and the second one takes longer to learn. It is not eluding you because you lack talent. It is asking you to see the whole person, not just the numbers. Most plans fail because they ignore what someone actually eats, when, and why. So start there. Before you write a single line, ask the person to walk you through one ordinary day of eating. Build the plan around what is already true for them, then change one thing at a time. You will get better at this the way you get better at anything: by watching what happens after. A plan is a guess until real life tests it. Treat each one as an experiment, not a verdict on your skill. The craft is in the adjusting.