Christians believe that Jesus instituted Communion at the Last Supper. In fact Jesus had just taken the middle matzah called the Afikomen from the single stack of three matzah during the Passover observance. He broke that matzah and then equated that unleavened bread to His body and poured the wine and equated that to His blood.
Seems simple enough but not for many professing Christians. Catholics believe the bread and wine actually becomes the physical body and blood of Jesus and those Christians who do not believe that are denied Communion in a Catholic Church. A local Catholic Bishop may make an agreement with a local Orthodox Bishop to extend Communion to Orthodox Christians. However, Local Orthodox Bishops will not comply. The Orthodox Church does not permit non-Orthodox Christians to take Communion in an Orthodox Church and the Orthodox Church forbids Orthodox Christians from taking Communion in non-Orthodox Churches.
Likewise Catholics are forbidden from taking Communion from non-Catholic Ministers. The Catholic Church considers taking Communion in a non-Catholic or Orthodox Church to be a grave evil and mortal sin. The Catholic Church considers their Bishops and Orthodox Bishops to have a direct line succession going back to the Apostles. Since non-Orthodox and Catholic Pastors cannot claim that line of succession they are considered illegitimate to offer Communion according to the Roman Catholic Church.