It’s perfectly fine; ignore this error. It does not affect functionality of contact form.
This message is displayed because we use a custom shortcode to dynamically replace the “To” address with the email of the listing owner on which the contact form is displayed. Without this, we could only set one email for all the forms, meaning only you, as the site owner, would receive the emails instead of the owners of the listings.
This solution requires the Dynamic Field add-on for Contact Form 7. The custom shortcode added via this plugin is not recognized by CF7 as valid code, which is why it shows this error.
If your form doesn’t you can be sure that this error does not cause that. You should check SMTP configuration instead:
Having problems with your WordPress site not sending emails?