What is the ruling on someone who, out of ignorance, omitted a letter from Al-Fatiha during prayer, then later became aware of it? Does he have to repeat the previous prayers, and does the saying of Allah Almighty, "Allah has forgiven what is past," apply to him?

What you did by deleting the letter "ل" (lam) from the word "المستقيم" (al-mustaqeem) in Al-Fatihah is considered a clear, manifest error (laḥn jalī) that invalidates the prayer. You must make up all the prayers you performed with this error if you are able to pronounce it correctly and have learned to do so. The Malikis hold that an unintentional error does not invalidate the prayer. So, if your error was due to ignorance, you are not required to make up the prayers. Sheikh Al-Islam Ibn Taymiyyah also held that there is no obligation to make up prayers for someone who omitted a pillar or an obligation due to ignorance. Nevertheless, making up the prayers is more cautious, to avoid scholarly disagreement.