I'm having the exact same issue. I'm using a MacbookPro, OS X version 10.10.5, Firefox Quantum 62.0, Airy 3.5.197 Personal License. I couldn't download the video in the YT link below on September 6th, so I updated Airy to the current version (3.5.197) and retried and it successfully downloaded this video and several others. Then on September 7th, 8th and 9th I was getting the same error message on all YT videos I tried to download.
On September 9, 2018 I tried downloading this exact same video to see if perhaps YT had made changes that would not allow me to download videos. I once again got the same message:
"Some links are not available We processed your links, but some videos cannot be downloaded for various reasons. These links were skipped. Skipped 1 of 1 links."
There seems to be a pattern... the YT videos that produce this error message have recorded music. When I try to download a less popular YT video w/o recorded music it successfully downloads. I think YT made changes sometime between Sept 6th - Sept 7th that is preventing Airy from downloading videos that could previously be downloaded.