No you cannot sit for exam , yes can enroll for GST practitioner
As per Notification 3/2019 CT , the council has extended the 18 months to 30 months to pass the Exam for those who has enrolled as TRP/STP.
So last exam which is going held on December 2019 is a last chance for TRP/STP. So Exam for Graduate enrolled will be appear for Exam conduct in June 2020.