The red "Practitioner is not available at this time" that can appear in the bottom left corner of the screen is shown when you are trying to book a service, block or break during a time that the staff member is not available.
It is not possible to double book staff
It is not possible to double book a practitioner for any event in the calendar.
E.g. If staff have a lunch break in the diary and you wish to block them out for the whole day, you will receive this error as they cannot have a block booked at the same time as their lunch break.
In this example, once selecting "Block out whole day", next click "Block out whole day except appointments" - this will fill the available gaps with a block avoiding the double bookings.
Staff must be available for booking blocks or breaks
It goes without saying that staff must have a column in the calendar and it be free for the event you are trying to book. If it runs outside of their working time, you will see a warning about this that can usually be overridden.