代君之禮 The Rites Owed To A Sovereign
行者方才立起身來，對眾道：「你王如何不來？」眾臣道：「我王身虛力怯，不敢乘輦，特令臣等行代君之禮，拜請神僧也。」行者道：「既如此說，列位請前行，我當隨至。」行者方才立起身来，对众道：「你王如何不来？」众臣道：「我王身虚力怯，不敢乘辇，特令臣等行代君之礼，拜请神僧也。」行者道：「既如此说，列位请前行，我当随至。」
逐句解 Line by line
Only then did Pilgrim rise to his feet, and he said to them all, "Why has your king not come?"
方才 is 'only at that point'. The narrator marks exactly how long he stayed seated — until the memorial had been delivered in full.
The ministers said, "Our king is weak in body and low in strength and dares not ride in the carriage. He has expressly ordered us to perform the rites in his stead and bow our invitation to the Divine Monk."
乘 is chéng, to ride. 行 is xíng, to perform a rite, and 代君之禮 names it exactly: the ceremony done in the sovereign's place.
Pilgrim said, "If that is how it stands, let your lordships lead the way and I shall follow."
當 is dāng, marking what he undertakes to do. 隨至 concedes the point without a scrap of graciousness: he will come, and he will come behind them.
The question is the whole negotiation. Pilgrim demanded that the king fetch him in person; a court has arrived instead; and rather than accept or refuse he simply asks why the terms were not met.
The answer satisfies him because it concedes the principle. 代君之禮 means the ministers are standing in for their sovereign, so the bows they have just made are the king's own bows. Pilgrim has been fetched by a king, technically.
He gets everything he asked for on a legal fiction, and goes quietly.