What is the expiation for someone who has not prayed two or three consecutive Fridays?

Friday prayer is an obligatory duty. Allah the Almighty says: "O you who have believed, when [the adhan is called] for the prayer on the day of Jumuʿah, then proceed to the remembrance of Allah and leave trade." The Prophet (peace be upon him) said: "Whoever abandons three Jumu'ahs out of negligence, Allah will seal his heart." The expiation for this sin is sincere repentance, consistently reciting the Quran, engaging in remembrance (dhikr) and seeking forgiveness (istighfar), performing obligatory prayers with the congregation, and increasing in piety.