What does this ERROR message mean?

This error only occurs when the nodetool repair service is running in preview mode. In preview mode, the repair service is trying to calculate and estimate how long a nodetool repair will take if it were run on the current node. It runs through the sstables for the partition key ranges as if it were running a repair without streaming any data but it does verify whether or not a nodetool repair is necessary.

This error message means that the nodetool repair --preview failed while it was running and trying to check the differences between the replicants in the cluster.

Why does this ERROR occur?

If there are active compactions running or incremental repairs are running during the nodetool repair --preview, this error could be thrown. This error could be thrown if a node holding the replicant data for the partition range is busy or otherwise not available during the nodetool repair --preview.

How do you fix this ERROR?

To prevent this error, be sure to turn off the OpsCenter repair service so it doesn't conflict with the nodetool repair --preview process.