What does this error mean?

This error means the LifecycleTracker could not mark sstables compacting.

Why does this error occur?

If the PendingAntiCompaction.acquireTuple receives a null LifeCycleTransaction, the error is thrown.

How do you fix this error?

In theory, this should not happen unless a bug is introduced. The code that throws the error explicitly runs with all compactions stopped, so Cassandra should always be able to mark the SSTables compacting.
If you encounter this error, you'll have to check the error for details (but other than that, you're probably good for restarting the repair).