Here "Sheol" and "death" refer to "dead people." AT: "For those in Sheol do not thank you; dead people do not praise you" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Hezekiah repeats this phrase to emphasize that only a living person, not a dead person, can give thanks to Yahweh. (See: [[rc://en/ta/man/translate/figs-parallelism]])