Flask "Error: The file/path provided does not appear to exist" although the file does exist
The werkzeug version is not suitable for flask. To address this problem, you need to upgrade the werkzeug, use:

$pip install werkzeug --upgrade
I use export FLASK_APP=flask_app and then do flask run but I get the error:

Error: The file/path provided (flask_app) does not appear to exist.  Please verify the path is correct.  If app is not on PYTHONPATH, ensure the extension is .py

However, the file does exist and is even in the present working directory. Using the complete path to the file does not work either.