Optus, I can't register for My Account. system keeps telling me my Plan85 mobile number is not allowed an account. Strange becausew the Optus App. when it was working, suggested I could check my post paid account/data balance!!
It's possible that you've been registered for My Account at point of sale.
For example, it's common practise for our retail staff to register customers upon signing up.
Have you tried the forgotten username or password option? I'd try entering your email address; you may receive an email with instructions explaining how to reset your password.
Otherwise, I'd jump onto Live Chat.
The agent can register your service manually from their end.
Personal → Mobile → General & Billing