What does this ERROR message mean?

If you are using the preview repair feature while an incremental repair is running, the results of the preview repair may not be accurate.

Why does this ERROR occur?

The preview repair feature was added in CASSANDRA-13257. It allows you to estimate how long a repair might take. If you are using this feature while an incremental repair is running, you will get this error. This error just points out that the estimate from the preview repair may not be accurate if incremental repair is running.

How do you fix this ERROR?

Disabling incremental repair while running a preview repair will make this error go away. Alternately, you can just accept that the preview repair results may be somewhat inaccurate.