SF6 is known but SCl6 is not.

Fluorine is the strongest oxidizing agent and it can oxidize sulphur to its maximum oxidation state +6 to form SF6. Chlorine is not a good oxidizing agent, it cannot oxidize

sulphur to its maximum oxidation state. Chlorine can oxidize sulphur to only +4 oxidation state. Hence it can form SCl4 but not SCl6.