I think it's an issue with the website. I checked at midnight. Wish there was some way to complain about the Home Office to someone. This is the reply I have got from them after sending my email, hardly helpful!
Thank you for your email correspondence of 1 June.
This inbox is only for technical support enquiries for applicants applying online within the United Kingdom via GOV.UK link https://www.gov.uk/visas-immigration
There is currently no reported problems with our online booking service.
Appointment availability is only up to 45 days, we are unable to confirm current availability.
The system will automatically take you to the nearest available date (where available), if the date you require is not available.
If there are no timeslots listed under ‘Available appointments from the date you have selected’ this would indicate, no availability.
Unfortunately we are unable to book, cancel or reschedule appointments.
We can only advise to keep checking.