Systems such as Oracle Linux 7 where SELinux is enabled and the security policies are set to "Enforcing" will restrict Fn from running containers and mounting volumes.
For local development, you can relax SELinux constraints by running this command in a root shell:
setenforce permissive
Then you will be able to run fn start
as normal.
Alternatively, use the docker-in-docker deployment that a production system would use:
docker run --privileged --rm --name fns -it -v $PWD/data:/app/data -p 8080:8080 fnproject/functions
Check the operating options for further details about this.