SINGAPORE—China has cast itself as a champion of free trade and multilateralism in the “America-first” era, but the world’s second-largest economy is finding that turning such talk into outcomes is substantially more difficult.

At meetings of global leaders in Singapore, negotiators working on a sweeping, 16-nation trade pact backed by Beijing said Wednesday they couldn’t finish the deal this year, citing its size and the complexity of talks among a range of economies.