I assume you are referring to Mobile Application Management, the setting that disables personal backups, and you want to know if it is working, by reviewing an audit log to see blocked attempts by users to use the personal backup feature? Is that correct? As far as I can tell, it does not look like MAM policy violations are logged. The closest documentation I could find is this article which appears to just log the value of the setting for a particular application managed by MAM.
https://docs.microsoft.com/en-us/intune/app-protection-policy-settings-log
