Rashi on Leviticus 27:29/רש"י על ויקרא כ״ז:כ״ט

1'כל חרם אשר יחרם וגו‎ NONE DOOMED WHO SHALL BE DOOMED OF MAN, [SHALL BE RELEASED] — This means, if a person is going to be executed and someone says, “I take upon myself to pay his ערך” he has said nothing (his vow is of none effect) (Arakhin 6b).

2מות יומת, you see, he is going to be put to death, and he therefore cannot be redeemed, — he has neither a market-value (דמים) nor an ‎ערך.‎