Hello student i am sorry to inform you but you cant apply for CBSE board examination as a private candidate as its for student who could not passed in 1st and 2nd compartment examination. refer to the link mentioned below for ore information - http://cbse.nic.in/newsite/attach/FINAL_LOC%20FOR%20COMPTT%202020_PRIVATE.pdf

However you can appear in board examination on demand through NIOS

Feel free to comment if you have any doubt

Good luck