Testing Java lambda function via API Gateway: Unable to determine service/operation name to be authorized

I'm playing around with API Gateway. Basically, I have a simple java code that aims to return a greetings message:

public class Greetings implements RequestHandler<GreetingsRequest, String> {

    //enable pretty print JSON output
    Gson gson = new GsonBuilder().setPrettyPrinting().create();
    
    public String handleRequest(GreetingsRequest input, Context context) {

        LambdaLogger logger = context.getLogger();
            
        System.out.println("Welcome to lambda function");
        
        // log execution details
        logger.log("ENVIRONMENT VARIABLES: " + gson.toJson(System.getenv()));
        logger.log("CONTEXT: " + gson.toJson(context));
        
        
         // process event
        logger.log("EVENT: " + gson.toJson(input));
        logger.log("EVENT TYPE: " + input.getClass().toString());
        
        return "Hello " + input.getName();
        
    }

}

I've attached to the lambda function a role with the following characteristics:

4 default policies (AmazonAPIGatewayInvokeFullAccess, CloudWatchFullAccess, AmazonAPIGatewayAdministrator, AWSLambdaBasicExecutionRole) and a custom one (lambda_execute).

Role's Trust Relationship:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "",
      "Effect": "Allow",
      "Principal": {
        "Service": [
          "lambda.amazonaws.com",
          "apigateway.amazonaws.com"
        ]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

In relation to the custom policy "lambda_execute":

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "lambda:InvokeFunction",
            "Resource": "*"
        }
    ]
}

In relation to the API Gateway:

The resource:

The method request:

The integration request:

When I am testing out the resource, the following message is sent out:

<AccessDeniedException>
  <Message>Unable to determine service/operation name to be authorized</Message>
</AccessDeniedException>

Anyone could point me out what I'm missing or doing wrong? Tks so much in advance.