$SF_{6}$ is known but $SCl_{6}$ is not. Why?

(N/A) This is due to $(i)$ Small size of fluorine and $(ii)$ High electronegativity of fluorine.
Fluorine,being highly electronegative,can oxidize sulphur to the $+6$ oxidation state and can be accommodated easily around the sulphur atom due to its small size.
Chlorine,on the contrary,has a larger size,which leads to significant inter-electronic repulsions between the chlorine atoms when six of them are bonded to a single sulphur atom. Thus,$SCl_{6}$ does not exist.