Google and YouTube are interested in watching videos on their site only, so they do not download videos directly from YouTube. But that does not mean it's impossible. You just have to follow a few simple steps to download the video from YouTube.
1. Download and install the 4K Video Downloader application. It is compatible with macOS, Windows and Linux.
2. Copy the video link from your browser.
3. Click the "Insert URL" button in the 4K Video Downloader program
4. Select the desired quality in the download window and press the "Download" button.
5. You just have to wait for the download to finish to watch your video.
You can watch the video guide "How to use 4K Video Downloader".