Hello student yes you can carry forward your December 2020 exam session to June 2021 exam session as the ICSI has decided to allow such students. You can also carry forward your examination fee to June 2021 students need not to pay the examination fees in June 2021.

For more details refer to the link mentioned below - https://www.icsi.edu/media/webmodules/Announcement_carry_forward_june_exam_fees_to_dec20examsession.pdf

Feel free to comment if you've any doubt

Good luck