Version 4.0 of the CFM-68K Runtime Enabler has been included in the System resources of Mac OS 8. You do not need to include the separate CFM-68K Runtime Enabler in the Extensions folder. By marking it with a red X, system software is indicating the extension is not necessary and therefore not loading.
Your applications should work correctly, since the system software contains the software already. Some applications may not realize that Mac OS 8 included this extension, and may install the CFM-68K Runtime Enabler during installation. In any event, you may throw the CFM-68K Runtime Enabler in the Trash after installing Mac OS 8.