I have just called them and asked to speak with a supervisor. She confirmed me that the scheduling will only be possible after Sep 23rd.
That's funny

The agent misunderstood because otherwise, that either means you can't take GMAT online again if you took it pre-Sept 23 OR that you can take it twice after Sept 23 both of which are wrong.
Have you tried scheduling a retake and not able to? Is that why you contacted the agent?
mateuslmachado
I was checking with Pearson agent if it is already possible to schedule a retake. He told me that retakes are only for exams schedule after Sep 23rd. Very wired.
"Please be informed that effective Sep 23, 2020 you will be entitled for 8 life time attempts including both the exam modes with a maximum of 2 attempts for online exams and 6 (if 2 attempts were for online mode) for test center mode with 5 attempts in a rolling 12 months period. You can also schedule 2 exams with a gap of 16 days.
Sunny at 12:02, Sep 18:
Please be informed that for any online exam scheduled before Sep 23, 2020, you will not be able to retake the exam.
Sunny at 12:03, Sep 18:
In such a case, you can retake the exam only if you had faced any technical issues during the exam.
Sunny at 12:10, Sep 18:
Please be informed that as per the recent update, you can retake the online exam only if the original exam has been scheduled starting Sep 23, 2020."