Hi there. All bookings need to be made through your MyACCA account, including the SBL exam. It may be that you received an exemption before the introduction of SBL, or perhaps you're referring to a mock exam run by Kaplan. If you can provide more details we can look into it further.
But for now, you will need to access the exam booking system by logging into MyACCA. Further guidance can be found on the ACCA website - click here for more info.
Hope this helps :)