what is the exact reason for "nagyon jó vagyok" not being accepted? (i mean, what does the L in the end means?)
"Jó" is an adjective (means "good") and "jól" is an adverb (means "well"). Adverbs usually get an -n/-an/-en ending (-ly in English), they get -l/-ul/-ül only rarely.
Hogy vagy? Jól vagy? = "How are you? Are you well?"
Nagyon jól vagyok. = "I am very well."
Jó vagy ebben a játékban? = "Are you good at this game?"
Nagyon jó vagyok. = "I am very good."