Mark 16:9-20 (KJV)
9 Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.
10 And she went and told them that had been with him, as they mourned and wept.
11 And they, when they had heard that he was alive, and had been seen of her, believed not.
12 After that he appeared in another form unto two of them, as they walked, and went into the country.
13 And they went and told it unto the residue: neither believed they them.
14 Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen.
15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
17 And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
18 They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
19 So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God.
20 And they went forth, and preached every where, the Lord working with them, and confirming the word with signs following. Amen.

The New International Version says this about the about the passage …

[The most reliable early manuscripts and other ancient witnesses do not have Mark 16:9-20]

The earliest manuscripts available and other witnesses may not have this ending of Mark. However, Irenaeus (120-202 A.D.) mentioned in his writing “Irenaeus Against Heresies”, chapter X…

Also, towards the conclusion of his Gospel, Mark says: “So then after the Lord Jesus had spoken to them, He was received up into heaven, and sitteth on the right hand of God;” confirming what had been spoken by the prophet: “The LORD said to my Lord, Sit Thou on My right hand, until I make Thy foes Thy footstool.”

It needs to be considered that the early manuscripts may not really be the most reliable. We find that the most relied on and used manuscript evidence was what was used in the King James Version translation. Hence the reason why these later manuscripts were so much more widely accepted and abundantly copied. Undoubtedly, discussion will go on about this topic.