What does this error mean?

This error indicates that a transaction sync operation failed to write cached transaction data to disk.

Why does this error occur?

During compaction, Cassandra tracks SSTable files via a transaction mechanism. To guarantee persistence in the event of node failures, the transaction mechanism relies on transaction log files that are written to the SSTable directories. Those files are copied to multiple identical replicas on different disks. During a transaction, Cassandra needs to sync (write all cached data) to disk in the SSTable directories at different points in time. If this operation fails (I/O Errors, other denial errors), the error is logged.

How do you fix this error?

This error will require a deeper investigation. A support ticket should be filed with DataStax Support.