What does this error mean?

This error means an SSTable could not be added to the promoted level and will be placed back in Level 0 for a table running the LeveledCompactionStrategy.

Why does this error occur?

This error can occur if a compaction has promoted an overlapping sstable to the given level, or was also supposed to add an sstable at the given level. Since the Leveled Compactions are moving sstables from unrepaired to repaired, the SSTable in question would cause an overlap of data within partitions which is not allowed with LCS.

How do you fix this error?

There is nothing to fix here. This is a natural safety mechanism of LCS. Since the SSTable was placed back in Level 0, it is still readable and will participate in future compactions.