What is the ruling on an individual's prayer if he does not say "Ameen" at the end of Surat Al-Fatihah, whether he is a follower or an imam, and is it said outside of prayer?

Saying "Ameen" after reciting Surat Al-Fatihah is a Sunnah for the Imam, the one praying behind the Imam, and the one praying alone. If someone omits it, their prayer is still valid. However, if a Muslim omits it until they become occupied with something else, they have missed its reward. Saying "Ameen" is also recommended for whoever recites Al-Fatihah outside of prayer, but its recommendation is stronger within the prayer.