This add-on works only for a short time and even then only at the 360 resolution. Then it stops working, it says the size of the 360 file is zero. The downloads history just says it failed. If course if you try any other resolution it gives some flannel that YT changed something and so they need you to pay for the PRO version. I have seen this tactic on some VPN add-ons.

So it works fine for a short period or a few downloads at 360, then it stops and your only option is to pay them money. This is the message if you choose any other resolution or format

"Youtube switched to separate audio/video streaming technology for higher quality formats and no longer provides direct 720p (Since 24 September 2018)/1080p/mp3 streams. Converting these streams to downloadable formats for you needs powerful server infrastructure, therefore 1080p/mp3 downloading has been limited to PRO users."