ColdFusion 11: " Variable SESSION is undefined" Error
Continue troubleshooting by determining if it's a problem with your install vs. your code. I would try creating a new site in Apache, using the CF server configuration tool, and then creating a new project in CFB. Create an Application.cfc file and index.cfm. In Application.cfc make sure you have this.sessionManagement = true and this.clientManagement = true. In index.cfm just cfdump the session. If this results in success you can look further into your existing code. If it fails you might want to try reinstalling CF.
This can also happen if you accidentally set the timeout params for session vars in CFAdmin to 0. It won't be unlimited, it will make the session scope go away.