ALWAYS include stack traces and the track that was played (in case of an URL) when reporting an error.
If Maven is used for building, then this is likely a problem with the way Maven handles version conflicts. When there are two things which require different versions of a dependency, Maven uses the braindead strategy of choosing the one which is the least transitive. This way it will likely choose an older version than the one required by LavaPlayer, which will cause runtime errors.
Unfortunately it is not possible to currently change the version conflict resolution strategy in Maven, so the only solution is to check the dependency tree and manually declare a dependency on the highest version of the affected package that can be seen in the tree.
An alternative is to use Gradle, which has a sane default (and if necessary, highly configurable) version conflict resolution strategy.
This is just a wrapper exception around all other exceptions, check the full stack trace.
Did you register source managers? For playing remote tracks (URLs including YouTube):
AudioSourceManagers.registerRemoteSources(playerManager);
For playing local tracks (files on disk):
AudioSourceManagers.registerLocalSource(playerManager);
Make sure you do not convert the input you pass to AudioPlayerManager#loadItem to lowercase as YouTube URLs are
case-sensitive.
In the class that calls AudioPlayer#provide, record the number of times it returns a null. If the count constantly
increases over time (excluding around when a track starts when it is fine to happen), it might be LavaPlayer lagging
behind (possibly the CPU is under too much load).
Otherwise, it is an issue with either packet sending or network. Packet sending issues might be caused by garbage collection pauses in the JVM - for JDA this is mitigated by JDA-NAS. For other Discord libraries, you should compare with a test bot with JDA+JDA-NAS on the same machine to verify it is not an issue with the library.