When a link fails, it is almost always the video, not the converter. Here is every case, what it means, and whether anything can be done.
Only the uploader and invited viewers can watch. The audio is not publicly reachable, so no converter can fetch it. Nothing to do except ask the uploader to make it public or unlisted.
Removed by the uploader or by YouTube. The link points at nothing now. If you need the audio, check whether the creator re-uploaded it.
YouTube requires sign-in to verify age. Converters fetch anonymously, so the audio stream is blocked. There is no workaround here.
Some videos are blocked in certain countries. If the block covers the converter's region, the fetch fails. A link that works for a friend abroad may fail for you, and vice versa.
The stream is still being written; there is no finished audio to fetch. Wait for the stream to end, then convert the replay. See live stream to MP3.
Occasionally YouTube mutes or removes a video's audio after a copyright claim, or blocks the video entirely. The page may still load while the audio is gone.
Some YouTube Music tracks are restricted to subscribers. If the audio is not publicly reachable, the link will not convert. See Music to MP3.
Other fixes: troubleshooting.