"There is one thing you are missing." Here "lack" is a metaphor for needing to do something. Alternate translation: "One thing you need to do" or "There is one thing you have not yet done" or (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here the word "it" refers to the things he sells and is a metonym for the money he receives when he sells them. Alternate translation: "give the money to the poor" (See: [[rc://en/ta/man/translate/figs-metonymy]])