Whilst there may be a few reasons for this, the likely reason is that the specific time you are trying to book is not available.
This can happen when:
Somebody else just booked your slot 😅
The clinic recently changed their opening or closing hours
A host of other more technical reasons
What to do
The best course of action is to politely ask the client if you can try to book another time, preferably on another day that would suit them.
If you would like to get further assistance you can always drop us a message by clicking the chat icon below.