The website https://cbe.accaglobal.com/CBE/Coordinator.dll?login?alias=CBE cannot be accessed and you are unable to start exams.
SNA may have problems downloading access tokens.
A firewall may be blocking access to the ACCA CBE server.
Communication with the server may also be blocked by some other means.
Check you have access to the internet in general (navigate to another website).
Check that your centre has not blocked access the above address.