Flask "Error: The file/path provided does not appear to exist" although the file does exist
Please follow these steps:

Make sure you have already done with [pip install --editable . ].  where '.' represent the location of directory where your app is installed. e.g(flask_app)
Run python
It will open command line python interpreter
Try to import the flask app
If its there error, you will get the detailed error.
Try to fix that error.

I do ran into the same problem and followed the steps above and found that there is error in running code.  Interpreter is showing compile error.
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.