Alma 17:4, 8; “And they had been teaching the word of God for the space of fourteen years among the Lamanites, having had much success in bringing many to the knowledge of the truth; yea, by the power of their words many were brought before the altar of God, to call on his name and confess their sins before him. 8 And thus they departed into the wilderness with their numbers which they had selected, to go up to the land of Nephi, to preach the word of God unto the Lamanites.” – 91 BC

Acts 13:1-3; “Now there were in the church that was at Antioch certain prophets and teachers; as Barnabas, and Simeon that was called Niger, and Lucius of Cyrene, and Manaen, which had been brought up with Herod the tetrarch, and Saul. 2 As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them. 3 And when they had fasted and prayed, and laid their hands on them, they sent them away.”

Why were there missionaries in the Book of Mormon in the BC era?

The Bible tells us the first Christian missionaries were sent out after Jesus’ resurrection, not before His birth.