The option to upgrade will not appear if the deadline has passed or if the course has ended and been archived. A select few edX open courses are audit-only.
To be eligible to receive a certificate, a learner must pay and upgrade to the verified track before the course's upgrade deadline. The upgrade deadline is fixed by the course team offering the course and it cannot be extended.
To try for a certificate you may instead enroll again in a future session. Most edX courses repeat in new sessions, so we encourage you to check back in the course catalog.