flask.cli.NoAppException: Could not locate a Flask application. You did not provide the "FLASK_APP" environment variable and a "wsgi.py" or "app.py" module was not found in the current directoryのエラーでて対応した Python

python エラー
http://xxxxxxxxxxxxx/　にアクセスすると以下のエラー

原因は、app.py がなかったため。これまでmain.py でやってたからアクセスできなかった。
flask.cli.NoAppException: Could not locate a Flask application. You did not provide the FLASK_APP environment variable
and a wsgi.py or app.py module was not found in the current directory.