If for some people (like me earlier) the above answers don't work, I think the following answer would work (for Mac users I think) Enter the following commands to do flask run
$ export FLASK_APP = hello.py
$ export FLASK_ENV = development
$ flask run
Alternatively you can do the following (I haven't tried this but one resource online talks about it)
$ export FLASK_APP = hello.py
$ python -m flask run
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