I had problems using python youtube_downloader/app.py. I had to move app.py into parent and run as python app.py. I haven't looked into the app.py file so I'm not sure the source of the problem. This was in MacOS.
Here is my full install:
python3.10 -m venv venv
source venv/bin/activate
git clone https://github.com/neonwatty/youtube_downloader.git
pip install --upgrade pip
pip install --upgrade setuptools wheel
pip install -r requirements.txt
mv youtube_downloader/app.py ..
python app.py