And here is more I learned on the same subject: 'lock' is not implemented under Ubuntu or #slackware so 'tmux lockc' does not seem to work either. The utility 'vlock' does the same thing, but is not installed by default.
But installing 'vlock' does not make 'tmux lockc' work under Linux. ☹️
Under #OpenBSD and #NetBSD 'lock -p' uses your default password, but 'tmux lockc' work as expected (meaning, it asks for your login password). No '-p' means entering a separate password for unlocking.