I'm currently able to do my enrolment through the 9 digit student ID without any trouble. They updated it and there are some more tabs to be able to complete it.
Found a solution from the HSC 2021 Facebook by HyAnh Nguyen:
UPDATE ON USYD ENROLMENT
You can only proceed these steps after you accepted your offer at USYD and received the "Enrolment is now open" email.
TLDR: enrolment for units of study and changes to personal details etc... occurs in the...