作善可解 Only Goodness Can Undo It
行者聞言，大驚失色，再不敢啟奏，走出殿，滿面含羞。四天師笑道：「大聖不必煩惱，這事只宜作善可解。行者闻言，大惊失色，再不敢启奏，走出殿，满面含羞。四天师笑道：「大圣不必烦恼，这事只宜作善可解。
逐句解 Line by line
Pilgrim heard this, went pale with shock, dared make no further petition, and walked out of the hall with his face full of shame.
大驚失色, greatly startled and losing colour, is a fixed four-word phrase that survives unchanged into modern Chinese. 含羞, carrying shame, is a strong word to use of this character. He promised a whole prefecture rain before he checked, and he now has to go back down and say so.
The Four Preceptors said, smiling, "The Great Sage need not be distressed. This matter can only be undone by doing good.
作善可解 — doing good can undo it. Four characters, and they are the hinge of the chapter. 解 is to loosen or untie, the word for dissolving a knot or lifting a spell, and it is the same character that closes every chapter of this novel in the formula about the next instalment.
Pilgrim walks out ashamed, and the Preceptors are smiling — because they were never going to let him leave without the answer, and they have waited until he stopped being certain to give it.
Their line reframes everything. The three conditions are not a sentence to be served, they are a lock, and the key is not force. Pilgrim has spent this chapter deploying the only tools he has ever had — power, reputation, the ability to summon and command — and none of them has moved this at all. What will move it is something he cannot do himself.