Electron affinity of Fluorine is less than that of Chlorine because

Electron affinity is the amount of energy released when an electron is added to a neutral atom to form a negatively charged ion.
The electron affinity of fluorine is less than that of chlorine due to the small size of the fluorine atom. The addition of an extra electron to the valence shell of the fluorine atom produces a strong electron-electron repulsion, as a result, low energy is released which is responsible for the low electron affinity of the fluorine atom.