Newly installed Windows App Control agents on Windows 10 and Windows 11 desktops will fully initialize and then sit at 0% agent sync for long periods of time that can be days or longer.
ClearPass OnGuard is detecting the App Control agent as unsecure software and blocking the communication to the App Control Server.
Uninstall ClearPass OnGuard from the endpoints, or edit the ClearPass OnGuard policy to allow the App Control agent to function as an approved software.