Reproduce an interpreter mismatch, compare executable, pip and module paths, then verify the repair from the Python environment that runs the application.
Catch a missing package resource by inspecting a wheel and testing its import, version, data file and console command in a clean environment.
ModuleNotFoundError has a handful of causes: the wrong Python, an inactive virtual environment, the wrong folder, or a file that shadows a module. Check them in order instead of reinstalling.
Importing a file runs it. if __name__ == "__main__" keeps a file's script part from running when another file imports it. Use it with a main() function, and know the cases where you don't need it.
Python is probably installed. Your terminal just can't find it. Check the three possible causes in order, fix the right one on Windows, Mac or Linux, and confirm which Python you're really running.
pip freeze gives exact versions but hides what you chose. A hand-written file shows what you chose but isn't exact. For anything you deploy, keep both: one file for intent, one frozen for exact versions.