There are a series of questions on this site which are generally "Is there X on Mars?" "Are there sufficient Y to create Z on Mars?" And the answers are generally YES for the most part.
What is there that we don't expect to be able to find on Mars? I don't mean "we haven't found this yet, but we haven't really looked very hard", I mean more like "given the soil composition we have from Mars, Mars seems to be very poor in X, which is a pity because that means you can't do Y".
So, what is there that we would strictly have to import from off-planet to Mars given what we know currently?