I started cs50 in June 2017 and I have finished in December 2018. I submitted my final project and the form on January 17, 2019. I also uploaded the video. I would like to apply for the certificate but I am not seeing the gradebook tab which seems to be from where I can see my marks and request my certificate. What is the reason behind this problem.
This sort of question is hard to answer here on Stack Exchange.
Based on our comment discussion, can you please send an email to firstname.lastname@example.org with your github/edX usernames (and perhaps a link to this question) and someone on the team can investigate your grading status.