I’ve read in a few threads that it can take a few days after receiving AOR for the access to be opened for you to create the account. If you’ve received in the last couple days, that could be the issue.
Another common issue is when setting up the account, the number of applicants is entered incorrectly. # should be 2 (for primary applicant and sponsor) if there are any dependents in the application they would need to be factored into the number as well.
I’ve found this YouTube account to be very helpful, hopefully what he goes over in the video can resolve your issue: