fsdisk (and others?) not working even as superuser

Total noob:

I’m trying to read a drive with my files I’m migrating from mac, but before I can even address that, there seems a more serious problem in that fsdisk (which has worked only intermittently) is off again. I log in as superuser / root, I get the cursor thing to switch to #, but I still get ‘command not found.’ Some internet research suggests it’s not just this one command, and others may not work either.

So, obviously, I need this to work. Would so like for it to work. Thank you.