It seems like you may have attended a recent workshop or online classroom environment pertaining to OpenShift Conainer Storage. The lab command is actually a script built into the classroom lab environment. You won't find the lab command available on systems outside of the classroom environment.
DO280 v4.5 - Red Hat OpenShift Administration II: Operating a Production Kubernetes Cluster
Section 2.6: Guided Exercise: Introducing OpenShift Dynamic Storage
logged in as user "student" on system "workstation" you run
lab install-storage start
As a test I spun up the course and was able to run that command without any issues. Perhaps something has changed in your path variable from previous labs? Can you try running the command with it's full path:
/usr/local/bin/lab install-storage start
If that works I would just reset the student user's path by typing:
Let me know if that works.