"ra'wI'wI' vItoy' 'e' vItIvbe'."
Translation:I don't enjoy serving my commanding officer.
I remember looking this up early on in the course. The word ra' is the verb for "command" and thus ra'wI' essentially means, literally, "a person who commands". However, ra'wI' does not correspond to the military rank of "commander". For that, Klingon has the word la' (as in Riker la'). ra'wI', in contrast, really refers more to "commanding officer", which is what the course requires as the translation.
I will leave it to the experts to comment further.
The course had better accept commander, as that is the gloss given in The Klingon Dictionary. Your analysis is correct, but commander means one who commands, so it is a correct translation of ra'wI'.
I suspect it was marked wrong because he said like instead of enjoy. Like is parHa'; enjoy is tIv.