What does this error mean?

During a validation compaction, no stables could be found for the designated repair operation.

Why does this error occur?

During the validation compaction, Cassandra attempts to pull a list of SSTables that should be involved in the repair operation. If the list returns null, the error message is logged and a RunTimeException is thrown.

How do you fix this error?

This error should not get thrown unless there is a bug of some type that results in an empty list. If you encounter this error, please submit a ticket to DataStax Support.