The state monitoring module for IIS failed to find the specified site. This will prevent the state monitoring module from providing the latest state of the site.
The most likely reason for this error to happen is if the site was recently removed from IIS and the discovery rule for sites did not detect the change yet.
If you recently removed the site specified in the error message, no action is required. Once the site is removed from Operations Manager the next time site discovery rule run, you can resolve this alert.