Flask "Error: The file/path provided does not appear to exist" although the file does exist
This could be many reasons.
python2 vs python3 issue, 
pip2 install Flask vs pip3 install Flask issue, 
and (venv) virtual environment vs local environment issue. 
In my case, had to do the following to solve the problem: 

python3 -m venv venv
. venv/bin/activate
pip3 install Flask
export FLASK_APP=flask_app
flask run
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.