The New American Bible (Revised Edition) is the translation that I have; this is the only translation which is authorized to be used in Catholic worship in the United States. The NABRE offers this as the dialogue (Mark 10:21–22):
Jesus, looking at him, loved him and said to him, "You are lacking in one thing. Go, sell what you have, and give to [the] poor and you will have treasure in heaven; then come, follow me."
At that statement his face fell, and he went away sad, for he had many possessions.
There's no specific gloss in the NAB for this version of the story. There is, however, for the version in Matthew (Matt 10:16–30), which has the following notes:
[19:21] If you wish to be perfect: to be perfect is demanded of all Christians; see Mt 5:48. In the case of this man, it involves selling his possessions and giving to the poor; only so can he follow Jesus.
[19:23–24] Riches are an obstacle to entering the kingdom that cannot be overcome by human power. The comparison with the impossibility of a camel’s passing through the eye of a needle should not be mitigated by such suppositions as that the eye of a needle means a low or narrow gate.
Thus it appears that at least from a Catholic perspective, what the rich man was lacking was the detachment from the world that would allow him to follow Jesus unwaveringly. Jesus' prescription for this was brute force: "go, sell what you have and give to [the] poor" (Matthew 19:21), and the man apparently didn't feel that he could.
