What does this error mean?
This error indicates that a forced recycle of all commit log segments was attempted but a segment was still connected to non flushed memtable data.
Why does this error occur?
This is not necessarily an error in the actual sense of the normal write path for Cassandra. The error here is simply that a segment that was supposed to be recycled could not be since there was still memtable data attached.
How do you fix this error?
There is nothing to fix for this error. When Cassandra refuses to recycle a commit log segment tied to unfleshed data, it is maintaining the durability that it was designed for. The error message is simply a statement to alert that the flush did not complete as expected. You may try running nodetool flush for the specified table to force the data to disk, freeing up the segment for recycling.