This error is related with the mail() function of PHP that sends the email. By some reason the mail() it’s not enabled in your server. In that case my suggestion for you is contact your hosting provider support about this.
An workaround is use SMTP to send the emails. For it just uncomment the lines below:
You need get the correct settings for SMTP with your hosting provider.
Please, try that and let us know if you need further assistance.