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. Read more at - https://videodroid.org/pro_upgrade.html. Free users can download all formats in cache where some Pro users fetched the video/audio in past 24 hours, to test the feature you can try downloading popular videos.