"the version number" - file(s) not opened on this client.
What little I know of Perforce is it will only let the user work with files that are defined in the users client spec.

So I would guess this message suggests to me your client spec is not configured to include the files you are trying to ...

I am using Perforce on the command line and I have a question.

I have been asked to "revert" a change that I made

I figure that that is no problem. I go to a command line interface and I log into perforce. Then I type:

p4 revert "the version number" "the filename with path"

Of course I gave it ...