Jesus is talking to a group of people about what they as individuals should or should not do. All instances of "you" and "your" are plural. (See: [[rc://en/ta/man/jit/figs-you]])
Jesus uses a question to teach the people. Here to "add one cubit to his lifespan" is a metaphor for adding time to how long a person will live. Alternate translation: "None of you can, just by worrying, add years to your life. You cannot add even one minute to your life! So you should not worry about things you need." (See: [[rc://en/ta/man/jit/figs-rquestion]] and [[rc://en/ta/man/jit/figs-metaphor]])
A cubit, a measure of a little less than half a meter, is used here as a metaphor for a short period of time. (See: [[rc://en/ta/man/jit/translate-bdistance]] and [[rc://en/ta/man/jit/figs-metaphor]])