The phrase is not completed can be a passive verb phrase; but if it is, it is present simple, and so it would refer to an act of completing, not a resulting state; furthermore, since it is present simple not present continuous, in most contexts it would refer to something habitual.
So as a verb phrase, It is not completed means something like It does not (generally, or normally) get completed - something that might be said, but not very commonly. I haven't been able to think of a plausible example of it.
However, like many past participles, completed can also be an adjective, describing the state that results from completing. In that sense It is not completed yet is fine: it is not yet in the state "completed".
Complete is more common than completed as the adjective, but both are po9ssible, and in this context I can't find any difference in meaning.