The word "flesh," which is weak and then dies, is a metonym for human weakness and death. AT: "the Israelites were weak and would someday die" (See: [[rc://en/ta/man/translate/figs-metonymy]])
They wanted God to prove that he could do what he said he would do before they would believe him. See how you translated this in [Psalms 78:18](./017.md).