Jesus uses a question to teach his disciples. Alternate translation: "None of you can make your life any longer by being anxious!" (See: [[rc://en/ta/man/jit/figs-rquestion]])
This is a metaphor because a cubit is a measure of length rather than of time. The image is of a person's life stretched out as if it were a board, a rope, or some other physical object. (See: [[rc://en/ta/man/jit/figs-metaphor]])