Dash Error Loading Layout - Zappa deployment

I tried to use Zappa to deploy my dash app. Since it is a test, so the code is quite simple:
app = dash.Dash(name)
app.server.secret_key = os.environ.get(‘secret_key’,‘secret’)
server =app.server
app.layout = html.Div(“hello world”)

if name == ‘main’:

Above codes work pretty well, but after depolying by Zappa, it just returns that error.

Here is the Zappa setting:
“dev”: {
“app_function”: “app.server”,
“profile_name”: “default”,
“project_name”: “test2”,
“runtime”: “python3.6”,
“s3_bucket”: “zappa-riyic8qiv”,
“aws_region”: “us-west-2”,
“slim_handler”: “true”

I tried the solutions posted here (https://github.com/plotly/dash/issues/22), it still cannot solve my problem.
Any ideas why? Thank u