If you are just "scanning" the registry with CCleaner when you get this message, you should just ignore it (and stop using CCleaner registry tools).
There is no benefit to be gained by using registry cleaners, but there is a significant risk of harm, sometimes irreversible. See https://www.whatthetech.com/2007/11/25/do-i-need-a-registry-cleaner/
The problem appears to be related to an interaction between CCleaner and Norton Security. Read through this entire thread, noting particularly the post from moderator Nergal: https://community.ccleaner.com/topic/27495-registry/