There are 3 ways to solve this problem

Method 1
Sometimes the downloading failed because the network is not stable.
you can right-click the video in “Downloading” list and restart the download again later

Method 2
If method one doesn’t work, it may because the file you are trying to download already exists.
Just find the file from the output folder and move it to another destination or rename it, and then try to download again.

Method 3
If the above 2 methods cannot solve the problem, try to change resolution or format by going to “Settings > Format”. You can set video/audio format and clarify freely.