Cannot access S3 bucket using flask+lambda

I made a lambda function with the following code (runtime python3.7). The role of the lambda has "AmazonS3FullAccess". However, I got "botocore.exceptions.ConnectTimeoutError" at the line "ret= client.list_objects_v2(Bucket='myfiles');".

What is the problem? How can flask+lambda access S3 objects?

import awsgi
from flask import Flask
import boto3;

app = Flask(__name__)

def lambda_handler(event,context):
    return awsgi.response(app,event,context);

@app.route("/",methods=["GET"])
def index():
  s3 = boto3.resource('s3');
  client = s3.meta.client;
  ret= client.list_objects_v2(Bucket='myfiles');
  print(ret);
  return "hello world";