Why doesn't my knowledge base search work?

I'm trying to duplicate this lesson on Querying the Knowledge Base. I've followed the instructions but it's not working. Can anyone tell me why?

Here is what's happening.

https://ibb.co/tgBhwDx

I've built the KB using Voiceflow's documentation.

https://ibb.co/TxTqQkgV

I save the entire user reply to {last_utterance} in the Capture step.

https://ibb.co/fVhShF5f

I create a KB Search step to search the knowledge base using {last_utterance}.

https://ibb.co/3Y16hhbY

I use a Prompt step to generate an answer.

https://ibb.co/60N1Gzv2

Here's the prompt.

https://ibb.co/VcfPjyH2