What prevents us from attaining Nibbana? What prevents us from attaining Nibbana?
Is it the ignorance or the craving?

So, what should we focus on, in order to attain Nibbana?
Is it uprooting the craving or uprooting the ignorance?

Because of what reason the uprooting of craving happen?
and
Because of what reason the uprooting of ignorance happen?
It's a "false dichotomy" to ask, "Is it this or that?" -- because it's both: they're related.

Given the "three poisons" (ignorance, desire, and aversion) I think that ignorance is described as the "root" of the other two -- it's because of ignorance that there's desire.

The word "ignorance" has a range of meanings: What is the difference between moha (delusion) and avijja (ignorance)?

I think that the four noble truths talk about the cessation of craving; and that the doctrine of the twelve nidanas say that craving is conditioned by sensation -- that it's caused or fuelled by e.g. "delighting" in pleasant sensations, aversion to unpleasant sensations, ignorance during neutral sensations.

So instead there should be "just seeing" and so on.

Another (more complicated) answer is that it's caused by habits or habitual tendencies, different kinds of desires and ignorance and wrong views: see for example What is effluent?

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A more literal answer, I read that the 12 nidanas can be divided into groups:


Causal process of previous life:


Ignorance
Sankharas

Process of effect in the present life:


Consciousness
Name-and-form
Six sense-bases
Contact
Feeling/sensation

Causal process in the present life


Greed
Appropriation
Becoming

Process of effect of future life:


Birth
Death



According to that model it's the link between 7 and 8 which you can affect, i.e. that where it's possible to "stop the wheel" -- because that's the part of it which happens in the present.