Finally, got Flask site deployed on Azure..... There's probably nothing wrong. The only problem might be my rebellious inner self that giving me a hard time.... A programmer under Mac or Linux OS may prefer to use python and develop under Linux compatible environment; they may choose Visual Studio Code. I developed my Flask under Windows environment (no IDE), using CMD for git version control, or, at the end, I didn't know how to make a simplest webapp work on Azure, trying on Visual Studio Community 2019 for deployment. (only for ASP... too bad...) Finally, I installed Visual Studio Code for deployment.... It worked.... Nothing wrong with my testing code.... Anyway, nothing wrong is a good thing. A fulfilled day!