480p and 1080p are no longer available as standalone videos downloaded from youtube.com. YouTube switched to adaptive streams and uses separate files for video and audio, which need to be merged to get a video file.
One of the easiest ways to merge both video and audio is VLC player built-in feature:
1. Choose: Open multiple files
2. Add downloaded video and then press show more options.
3. Tick: Play another media synchronously
4. Add downloaded audio track
5. Press the arrow next to play and choose "Convert"
6. Add output file name and press "start"