If I’m correct, the Coptic Church isn’t in communion with the Pope. Why aren’t these Churches in communion and is the Coptic Church’s Communion valid?
The Coptic Church does have valid sacraments because they are an ancient Church that has preserved the line of Apostolic Succession. However, you are correct that they are not in union with the Catholic Church. Its split from the Catholic Church came in 451 when it refused to accept the Council of Chalcedon’s dogmatic definition of the union of the divine nature and human nature of Christ.
There is a Catholic Coptic Church that is in communion with Rome.
In 1741 a Coptic bishop in Jerusalem, Anba Athanasius, became a Catholic. Pope Benedict XIV appointed him Vicar Apostolic of the small community of Egyptian Coptic Catholics, which at that time numbered no more than 2,000. Although Athanasius eventually returned to the Coptic Orthodox Church, a line of Catholic Vicars Apostolic continued after him.