In the booking view page, if the booking is unpaid, you can click on Finance button and choose the Payment Reminder option.
In the opened pop up, you can set the date of the reminder and add a description for it.
You have access to the list of payment reminders from the Menu->Finances-> Guest Payments-> Payment Reminders.
When you click on Payment Reminder, a window pops up in which you can set the Reminder Date and add Description. Then, Save