To be eligible to receive a certificate a learner must pay and upgrade to the verified track before the course's deadline. The option to upgrade will not appear if the deadline has passed or if the course has ended and been archived.
In these cases it is not possible to earn a certificate in this session of the course. The upgrade deadline is usually 10 days before the end of a course, though it may be earlier in some courses depending on assignment deadlines.
To try for a certificate you may instead enroll again in a future session. Most edX courses are repeated.