RobH_en_tn/psa/118/003.md

Let the house of Aaron say

Here the word "house" represents the family and descendants of a person. This phrase refers to the priests, who were descendants of Aaron. Alternate translation: "Let the descendants of Aaron say" or "Let the priests say" (See: rc://en/ta/man/translate/figs-metonymy)

His covenant faithfulness endures forever

The abstract noun "faithfulness" can be translated with an adjective. See how you translated this in Psalms 118:1-2. Alternate translation: "He remains faithful to his covenant forever" (See: rc://en/ta/man/translate/figs-abstractnouns)