Hello.
If you are receiving emails after visitor submit the booking, so then you can ignore that warning at the Booking > Settings > Emails page.
Otherwise check these this:
Please note, Booking Calendar is use standard WordPress wp_mail function for sending emails. This function can be overridden by other plugins.
Please check this troubleshooting instruction http://wpbookingcalendar.com/faq/no-emails/
The most probably you need to check the point #5 from this instruction and install WP Mail SMTP plugin which is ‘sending emails via php’ option and it can resolve that issue.
Kind Regards.
