Translation:You need to eat more.
Why is 'you must eat more' incorrect? The sense in English is identical.
Maybe they want the verb 'deber' used for must.
Wouldn't that need to be "Usted tiene que comer más"
This sounds like an imperative statement. Which is 'necesite'. And 'necesite' is also present subjunctive. We are discussing NEED here.
Why "you have to eat more" is wrong ?