Plesk 17.8.x is running in a Virtuozzo container
In Advisor > Security > Use the KernelCare Extension it is offered to install or open KernelCare extension, even though it is not supported
If KernelCare is installed, the following message is shown in Extensions > KernelCare:
PLESK_ERROR: unsupported kernel, contact
helpdesk.cloudlinux.com for more info
Bug #EXTADVISOR-344 in Security Advisor Extension, which will be fixed in future updates.
The message can be safely ignored.