Nearly correct! You have an extra apostrophe in the word.
DajatlhlaH'a' breaks down into Da-jatlh-laH-'a' -- the apostrophe belongs to the -'a' yes-no question marker, not to -laH "able, can".
(And Klingon can't have more than two consonants in a row, counting Klingon letters such as ch and tlh as a single consonant, so -H'b- can't be possible. There are three exceptions but -H' is not one of them.)
So it would be tlhIngan Hol vIjatlhlaHbe' "I cannot speak Klingon."
The word for "not yet" is wej (which happens to sound like the number "three").
It's an adverb so it goes at the beginning of the sentence: wej tlhIngan Hol vIjatlhlaH "I cannot speak Klingon yet".
Note that wej already has in it the meaning of "not", so there's no -be' on the verb.