In order to checkout any service there must be an appointment created in the calendar. So, in order to checkout walk-ins you need to create an appointment choosing the services which have been performed and then carry out the checkout.
The same applies for scheduled customers, those who already have an appointment created in calendar but have received additional services which were not created before. To check them out as well, open the particular appointment, click ‘add next service’ and then choose the particular service. If you need to add multiple services just click ‘add next service’ again up until you have added all services.