Hello Aspirant

You should enter 2020 as the passing year in the SSC CGL application form.

Here's why:

Completion of Degree: You completed all requirements for your degree, including clearing backlogs, in 2020.

Eligibility Criteria: The key is to have completed your degree before the eligibility cutoff date for SSC CGL.

Also, as you have mentioned, your scorecard has 2020 as the passing year, so you anyway will have to mention it as 2020 because the authorities cross-check all the documents.