"You do not know what to say."
Translation:Vi ne scias kion diri.
That's assuming if I added a question mark at the end. Rather, this is a statement. It's the difference between "Kion vi ne scias diri." and "Kion vi ne scias diri?" I assumed the word order doesn't matter as the n- suffix is the key indicating it is the object of not knowing, rather than the structure of the sentence.
Despite the -n ending, word order does matter in Esperanto in many situations: Kion vi ne scias diri? = What don't you know [how] to say? [Tio,] Kion vi ne scias diri = That thing you don't know [how] to say. Diri kion vi ne scias = [To] say what you don't know. Ne vi diri kion scias = ?????
It sounds like a question anyway, even without a question mark. "Kion" is the object of "diri", and the object of "scias" is the entire clause "kion diri".
Maybe it is clearer if we translate it as "Vi ne scias tion, kion diri". This "tion" is here to introduce the subclause, but you can usually leave it out.
This construction makes less sense in English than it does in Esperanto, but it's generally accepted that "tio(n), kio(n)" means the same thing as simply "kio(n)" in this context. The sentences you provide above have the same meaning; one just sounds more natural in English because we don't have this construction. "ti-, ki-" constructions are very common in Esperanto. I've had it explained to me that using simply "kio(n)" is actually an abbreviation for the full "tio(n), kio(n)," although I'm not sure how true that is.
This one gave me trouble back in the beginning too. What worked for me was thinking of the word "sits" then leaving out the 'i'. This helped me to set into my head the fact that there are two sounds.
Scii is not the only place you will find this combination; scienco, sceno, sceptero & sciuro (science, scene, scepter, & squirrel) all come to mind & the combination also shows up in the middle or end of other words, so hammering away at this is needed.
I accidentally put 'ke' instead of 'kion'. Native Spanish leaking through