Our Blessed Saviour said,

“You still lack one thing”

What did He mean to one of the most “law obeying young man”?

Let Scripture answer (please see below):

“Jesus looked at him, loved him, and said to him, “There is one thing you lack: Go, sell everything you have and give to the poor, and you will have treasure in heaven. Then come, follow Me.” (Mark 10:21)