Why is "Lernst du Deutsch?" incorrect?
It isn't. If it's not accepted, I'll bet a lingot it's a Pearson sentence.
The only options to choose from is ‘Lernt ihr Deutche’ Not ‘Lernst ‘ still correct?
Deutche is never correct, but Lernt ihr Deutsch? is a correct translation if you are speaking to several people at once.
du lernst for one person, ihr lernt for several people.