Je ne sais pas ce mot

might have been used several centuries ago to state you don’t know the word meaning. It might still be used in some contexts, for example a student has to learn a list of words and one of them is not known by him/her.

Je ne connais pas ce mot

usually means you never heard about it.

You might know a word without knowing what it means though:

Je connais le mot agape, mais ne sais pas ce qu’il veut dire

I would then translate:

I don’t know that word

with either (existence)

Je ne connais pas ce mot

or (meaning)

Je ne sais pas ce que ce mot veut dire.