Fluorine has the highest electro negativity among the halogens 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 do not show positive oxidation state

chlorine can accommodate an electron better than fluorine by utilising its large atomic size and vaccant d orbitals

small size, high electron density favours  addition of an electron to fluorine than chlorine