This double negative emphasizes that the priests and Levites were the only people whom the assembly was to allow into the house of Yahweh. Alternate translation: "The only people you may allow to come into the house of Yahweh are the priests ... serving" (See: [[rc://en/ta/man/jit/figs-doublenegatives]])
It may be helpful to state that command. Alternate translation: "All the others must remain in the courtyard, obeying what Yahweh has commanded" (See: [[rc://en/ta/man/jit/figs-explicit]])