Issue with configuring API gateway as a trigger for lambda function

The issue you're experiencing with the empty URL in the Lambda function trigger configuration when using Terraform is likely due to a missing or incorrect configuration in your Terraform code. When setting up an API Gateway trigger for a Lambda function, there are a few key components that need to be properly configured:

1. API Gateway Resource: Ensure you have defined the API Gateway resource with the correct path.

2. API Gateway Method: Configure the HTTP method (GET, POST, etc.) for your API endpoint.

3. Integration: Set up the integration between the API Gateway and the Lambda function.

4. Lambda Permission: Grant API Gateway permission to invoke your Lambda function.

5. Deployment: Deploy your API Gateway to make the changes effective.

To resolve this issue:

1. Review your Terraform configuration to ensure all these components are properly defined.

2. Check that you're creating an API Gateway deployment resource in your Terraform code.

3. Verify that you've set up the necessary Lambda permissions for API Gateway to invoke your function.

4. Ensure that your API Gateway and Lambda function are in the same region.

5. After making any necessary changes, run terraform apply again to update your resources.

If the issue persists, you may want to use Terraform's output values to display the API Gateway URL after deployment. This can help you confirm whether the integration is set up correctly, even if it's not showing in the Lambda console.

Remember that the Lambda console might not always reflect the exact state of resources managed by Terraform. The empty URL in the console doesn't necessarily mean the integration isn't working. You can test the API endpoint directly to verify functionality.

If you continue to face issues, consider breaking down your Terraform configuration into smaller parts and applying them incrementally to isolate where the problem might be occurring.
Sources
Creating api gateway api from openapi specification with lambda integration does not create triggers | AWS re:Post
AWS Lamdba function API trigger shows an error and is not accessible | AWS re:Post