What does this error mean?

This error means an obsolete SSTable could not be deleted.

Why does this error occur?

When Cassandra detects that an SSTable has become obsolete (for example after a compaction) and is not referenced anymore by any process, it will attempt to delete the SSTable. If an I/O issue prevents the SSTable from being deleted, the error message is logged.

How do you fix this error?

You should not need to fix this error. As mentioned in the error message Cassandra will automatically attempt to retry the deletion after and old generation collection and on restart. If the error occurs regularly, investigate the device health.