From the DVSA:

We wrote to you on 6 May 2021 to let you know we are making changes to the theory test service for England, Scotland and Wales, including changes to the theory test booking service.

The new theory test service will start from Monday 6 September. As part of the new service, a new booking system was due to open on 8 July.

Unfortunately, this has been postponed to 19 July to allow time for final testing before we launch. We are sorry for any inconvenience this may cause.

You can continue to book tests for dates on or before 3 September only, using the current booking system. Theory test dates for Monday 6 September onwards will only be available when the new booking service opens on 19 July.

We will provide an update on the opening of the new theory test booking service and planned changes to theory test centre locations later this month.

Practical test bookings are not affected by changes to the theory test service.

Thank you for your patience.