Hi

CBSE rule states that any candidate who has appeared for 12th exams and is not happy with his/her score can appear for improvement exams in one or two subjects in the succeeding year only. If the candidate is under vocational stream he/she may reappear for the improvement of the subject in the main examination of the succeeding year or the following year provided they have not pursued higher studies in the meantime.

Now according to this rule since you had completed 12th in 2020 you were supposed to appear for improvement exam in 2021 only. The rules revised this year which states that students can give compartment exams to improve their scores so that they don't have to wait for a year to improve their scores is only applicable to students appearing this year in 2021 and succeeding years.

Hope this answers your query!