Hello Ashiffa,

Sorry to inform you dear but you can only re-apper for the improvement exam in the succeeding year only i.e. 2021 in your case.

See if you've passed your 12th in 2020 then you should filled the improvement form around August/September 2020 itself.

Drop a mail at CBSE or ask any of your friend/relative here to go visit their office regarding your situation.

For more information you can visit- https://news.careers360.com/cbse-improvement-exams-2021-check-class-10-12-date-sheet-eligibility-criteria-here

Hope you find this helpful.

All the Best!