What does this error mean?

This error means SSTable files for a transaction were not removed.

Why does this error occur?

During compactions, Cassandra tracks SSTable files using a transaction mechanism. When a compaction succeeds and the transaction is committed, the old SSTable files will have to be removed. If the compaction fails or is stopped and the transaction is aborted, then the newly created SSTables files will have to be removed. If some of those files cannot be removed (e.g. a file no longer exists;  IO problem) the error is logged along with information about the transactionLog and the IO error.

How do you fix this error?

There is no specific fix for this error. Further investigation will be needed. A support ticket should be filed with DataStax Support.

Note: The removal of leftovers is also trigger by tools like sstableutil --clean or nodetool scrub.