“And afterward he appeared...and he rebuked them…” This verse contradicts Luke 24:34 because when Jesus appeared to the disciples behind closed doors, they were already saying he had been raised, so why would he reprove them? The disciples did not believe the women, that is true, but in the biblical culture, the testimony of women was not allowable in court. The disciples did believe Peter and the two men on the road to Emmaus, so Jesus would not reprove the disciples for “not believing those who saw him after he was raised.” They did believe the three men whose testimony was credible in that culture. Furthermore, reproof certainly does not seem to be the tone of Jesus’ communication with the disciples according to Luke 24:36-49 and John 20:19-23. The best explanation is that the closing section of Mark is not original.
[For more information on Mark 16:9-20 not being original, see commentary on Mark 16:9.]