First, setup the Email SMTP and/or SMS details in Business settings.
Go to Notification Template
Under Customer Notification you will see New Sales notification.
There you will find the option to enable Auto Send Email & Auto Send SMS, enable it and save.
Now when ever you make a sales a Email or SMS notification will be send to customer.
Important NOTE:
If there is any error in Email or SMS configration then sales will not get submitted, so test it properly.
Email notification will only get send in case the customer have a valid email present
Similary SMS will only get send if the mobile number is present.
To enable automatic WhatsApp notifications, enable the ‘Auto send Whatsapp Notification‘ check box under Customer Notification > New Sale.