Impossible to analize the link: https://www.youtube.com/watch?v=LwpJjYxl3Gw

If you have an antivirus program of a firewall, please try to disable them and after that paste link to a video in 4K Video Downloader.

If this fixes your issue, please add our application into the whitelist of your antivirus\firewall in order to stay safe while browsing the net and also download without interruptions.
If it doesn't help, try changing directory folder in Smart Mode settings, then try to download something (both with Smart Mode turned on and off).