Your Flask application should be something like below:

import os

from flask import Flask

app = Flask(__name__)


@app.route("/")
def hello_world():     
    return "Hello World!"


if __name__ == "__main__":
    app.run(debug=True, host="0.0.0.0", port=int(os.environ.get("PORT", 8080)))

See this official documentation for step by step instruction: Deploy a Python service to Cloud Run

There is a plugin called: Cloud Code IDE plugin which makes the test and deployment easy. I am using it for VS code, once the initial setups and permissions are taken care, few clicks, you will be able to run locally, debug and deploy Cloud run services from your local instance.