en_tn_lite_do_not_use/deu/23/06.md

You must never seek their peace or prosperity

Possible meanings are 1) "You should never make a peace treaty with the Ammonites and the Moabites" or 2) "You must not do anything to cause things to go well for those 2 people groups to enable them to prosper" .

during all your days

This is an idiom. Alternate translation: "as long as you are a nation" (See: rc://en/ta/man/jit/figs-idiom)