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?