Installing Modules :
$ pip install colorama
$ pip install pytube
$ pip install pytube3
if above line (pytube) doesn't work try following :
$ pip install git+https://github.com/pytube/pytube
$ git clone https://github.com/GH0STH4CKER/youtube_video_downloader.git
$ cd youtube_video_downloader
$ python3 youtube_downloader.py