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You’ll need to contact CFA Institute (info@cfainstitute.org) and try to manually request that they process it for you, since the refund window has long passed for December 2020 cancelled candidates.
Call them up if they don’t answer their emails.
It’s not guaranteed that they will refund you though, since they’ve only agreed to refund you during the refund window. In fact, candidates have reported little luck in getting CFA Institute to refund them outside of the refund window. But you likely can register for a future (2022?) exam.
Good luck, and let us know how it goes!