Web App Demo: Unable to deploy Backend by one-click deployment to Google Cloud Run

Reproduce steps: by clicking the big button in readme.md

[ ✓ ] Pushed container image to Google Container Registry.
[ ! ] FYI, running the following command:
gcloud run deploy backend\
--project=solid-heaven-271414\
--platform=managed\
--region=us-west1\
--image=gcr.io/solid-heaven-271414/backend\
--allow-unauthenticated
[ ✖ ] Failed deploying the application to Cloud Run.
Error: reason=HealthCheckContainerError message=Revision 'backend-00001-afy' is not ready and cannot serve traffic. The user-provided container failed to start and listen on the port defined provided by the PORT=8080 environment variable. Logs for this revision might contain more information.

even I specify the port 8080 and deploy again it fails, while Frontend has no issues of deployment

The log from the Cloud Run services is:
Error: '$PORT' is not a valid port number.