SF6 is known but SH6 is not known Lets analyze :
SF6 is known because Fluorine (F) is a strong oxidizing agent that means it under go reduction and oxidize Sulphur(S) to its maximum oxidation state i.e. +6.
Whereas H2 is a weak oxidizing agent, so cannot oxidize S to its maximum oxidation state of +6, hence does not form SH6.
F is the most electronegative element and that’s why it can use all the 6 electron of sulphur but hydrogen can’t do so as it is not much electronegative
So the answer to question
Why SF6 is known but SH6 is not known?
Since SF₆ is a strong oxidizing agent can exhibit oxidation number +6 whereas SH₆ being a weak oxidizing agent can’t expand oxidation number +6. That’s the reason its not known