Looks like your web hoster disabled the PHP function escapeshellcmd() on their servers. This is sometimes done citing security risks, and causes sending e-mails through PHP’s own mail() function (which your use of $kirby->email() invokes) to fail with above error.
To work around this, try using SMTP to send e-mail instead of the default configuration, as described here; you will have to find the correct SMTP server settings (and probably set up an SMTP account for sending) from your hoster.