What is wrong with saying that solid potassium chloride is composed of #KCl# molecules?
It is incorrect because potassium and chlorine do not react to form molecules. There is no such thing as a "KCl molecule".
Potassium has a valence of 1, and chlorine is significantly more electronegative than potassium. Therefore, the reaction between the two proceeds by potassium losing its outer valence electron to form a positive ion, and chlorine gaining the electron to complete its outer octet.
Two ions result, which, due to their charge, mean that KBr exists as an ionic lattice.
In order for a molecule to be formed, no electron transfer from one atom to the other would take place, instead there would be some degree of electron sharing. This does not happen with potassium and chlorine, so KBr is not composed of molecules.