Fluorine has the highest electronegativity among the group on the Pauling scale, but the electron affinity of fluorine is less than that of chlorine because

The atomic number of fluorine is less than that of chlorine

Fluorine being the first member of the family behaves in an unusual manner

Chlorine can accommodate an electron better than fluorine by utilizing its vacant 3d orbital

Small size, high electron density and an increased electron repulsion make the addition of an electron to fluorine less favourable than that in the case of chlorine