In Shia Islam, the 12th Imam, Muhammad al-Mahdi, is believed to be in occultation and will reappear at a time appointed by God to establish justice and peace on Earth. The Rapture is a Christian theological concept where believers will be taken up to meet Jesus Christ in the air.
Since these are distinct beliefs within different religions (Shia Islam and Christianity), there is no direct interaction or relationship between the events of the reappearance of the 12th Imam and the Rapture. They exist within separate eschatological frameworks.