I want to deploy the app to GAE.

I want to deploy the Python-created app to GAE, but it doesn't work because of the error.
You are about to deploy an app created with python3+flask to the Python37 standard environment of the GAE.

Error Contents

Beginning deployment of service [default]...
Created.gcloudignore file.See `gcloud topic gcloudignore` for details.
╔════════════════════════════════════════════════════════════╗
╠═ Uploading 7 files to Google Cloud Storage═╣
╚════════════════════════════════════════════════════════════╝
File upload done.
Updating service [default] ... failed.                                                                                                                                              
ERROR: (gcloud.app.deploy) Error Response: [7] Access Not Configured. Cloud Build has not been used in project xxxxx-xxxx before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/cloudbuild.googleapis.com/overview?project=xxxxx-xxxx then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.

Do you need cloudbuld?
I have to use cloudbuild, but it doesn't appear in other articles.Is this really necessary?https://qiita.com/kai_kou/items/775bcc058aaabbdff4e7

What is appengine_config.py
The document says that appengine_config.py should be placed in the root project directory, but is this also necessary for python37 standard environment?
https://cloud.google.com/appengine/docs/standard/python/getting-started/python-standard-env?hl=ja

Do you need the App Engine SDK for python?
I think I installed the Google Cloud SDK, but I don't remember if I installed the App Engine SDK for python, is there any way to check?Is this what I need to deploy the Python app to GAE in the first place?

Do I need to activate billing before deploying GAEs?

The App Engine application has been created. Can't I deploy it until there is a start button under and press it to set up credit registration?
It says that it was created, so I feel like I'm done setting up a GAE, but it's my first time touching it, so I don't know how it works.

As a beginner, there are many things I don't understand, so I can't deploy the application I created in GAE.
I would appreciate it if you could let me know.
Thank you for your cooperation.