We can't find the Get Customer Details action. Make sure it has been created and has an API Name of 'Get_Customer_Details'.

Agentforce – Create “Get Customer Details” Topic Action

Playground Issue: 

• Playground is not working related to https://trailhead.salesforce.com/promo/orgs/agent-customization-with-flows
• Register here - https://trailhead.salesforce.com/promo/orgs/learn-agentforce-for-employees-on-trailhead
• Reference: https://trailhead.salesforce.com/trailblazer-community/feed/0D5KX00000gSjCb0AK

Please also follow the below steps to avoid the error -

1. Setup → Agentforce Agents → CC Service Agent → Open in Builder → Deactivate.

2. Click the Experience Management topic.

3. Click the This Topic’s Actions tab.
4. Click New and select Create New Action
5. Set Reference Action Type to Flow.
6. For Reference Action, select Get Customer Details.
7. Leave the other options as is, and click Next.
8. Uncheck Show loading text for this action.
9. For experienceName, check Require Input.
10. For experienceRecord, check Show in conversation.
    
11. Click Finish.