The phrase "highest heaven" is an idiom that refers to heaven itself. The writer speaks to heaven as if it were a person and commands it to praise Yahweh. Alternate translation: "Praise Yahweh, highest heaven, as people do" (See: [[rc://en/ta/man/translate/figs-personification]])
The writer speaks to "the waters that are above the sky" as if they are people and commands them to praise Yahweh. The verbal phrase can be supplied from the previous line. Alternate translation: "praise him, you waters that are above the sky, as people do" (See: [[rc://en/ta/man/translate/figs-personification]])