HNO2 has the correct structure of HON=O, with the nitrogen having a lone pair of electrons.
The formal charge is calculated this way:
FC=valence e- – surrounding electrons.
Therefore, the N is surrounded by three electrons (1 from a single bond and 2 from a double bond) and a lone pair:
Formal charge of N = 5 – (3+2) = 0
The formal charge is zero.