For virtual environment:

Windows:

• Clone project to your computer
• Open project file with Pycharm IDE
• When the project is opened click File>Settings>Project:{YourProjectName}>Interpreter
• Click the gear icon and add new interpreter
• After interpreter is created, click the plus button and write Flask on the search bar.
• Select "Flask" package and click install package button

MacOS:

• Clone project to your computer
• Open project file with Pycharm IDE
• When the project is opened click Pycharm>Preferences>Project:{YourProjectName}>Interpreter
• Click the gear icon and add new interpreter
• After interpreter is created, click the plus button and write Flask on the search bar.
• Select "Flask" package and click install package button

After flask package is installed, first line in the app.py file has no longer red underline.