Chemistry-lattice enthalpy

if covalent bonds are stronger than ionic and NaCl has more covalent character than NaF. Then why does NaF have a more negative lattice enthalpy?

Don't worry about covalent character for this question as it is not relevant. NaCl and NaF have negligible covalent character. Think about the size of the attraction between the oppositely charged ions in the two compounds:

They both have Na+ ions
They both have negative ions with a 1 minus charge (Cl- and F-)
The Cl- ion is bigger than the F- ion
This means there is greater charge density (i.e. charge per unit volume ) on the F- ion so there is greater attraction to the Na+ ion, so lattice enthalpy of formation more negative (or lattice enthalpy of dissociation more positive.

Also, it is a bit dodgy saying "covalent bonds stronger than ionic bonds". Ionic "bonds" don't really exist- you are talking about the attraction between the + ions and the - ions i.e. lattice enthalpy.

(As a comparison strength of C-H covalent bond 412 Kjmol-1; Lattice enthalpy of ionic NaCl 770 Kjmol-1)

see attachment for more detail