plenv: no such command `shell'

[douglasdd]

Yes, this is technically user error: the shell has not yet invoked eval "$(plenv init - [...])"
...but we know what the user was trying to do, so we can & should provide a friendly/helpful error message.
Something like:

This shell is not configured to use plenv. 
To fix, run this command now (or add the command to your shell config and start a new shell):
    eval "$(plenv init -)"

(I'll try to send a MR, after vacation ;-)