This article applies to PRTG Network Monitor 12 or later, as well as to previous (deprecated) versions
Receiving the error message: 'root\cimv2' namespace is not defined in the WMI Repository
WMI Sensors: Invalid Namespace Error
root\cimv2 is the standard namespace for every query PRTG (and many other tools) uses in order to monitor performance counters. So if this is missing, your WMI system is definitely not working correctly.
Please ensure that the credentials you're using have Administrator rights on the monitored machine. If the error persists, please have a look at the instructions in this article: