en_tn_condensed/isa/40/06.md

All flesh is grass

The word "flesh" refers to people. The speaker speaks of humans as if they are grass, because they both die quickly. AT: "All people are like grass" (See: rc://en/ta/man/translate/figs-metaphor)

all their covenant faithfulness is like the flower of the field

The speaker compares the covenant faithfulness of people to flowers that bloom and then die quickly. The abstract noun "faithfulness" can be stated as "faithful." See how you translated "covenant faithfulness" in Isaiah 16:5. AT: "they quickly stop being faithful to the covenant, just like a flower of the field dies quickly" (See: rc://en/ta/man/translate/figs-simile and rc://en/ta/man/translate/figs-abstractnouns)

covenant faithfulness

Possible meanings are 1) covenant faithfulness or 2) beauty.