There is no regulation that states you can be forced to attend a board. However it is definitely in your best interest to attend for several reasons.
As an individual who was in this specific situation let me share with you my experience. First if given the order to attend and it is your place of duty then yes it is a requirement to attend. My BDE CSM gave me a counseling statement directing me to attend the Sergeant Audie Murphy board. So I told him you can make me show up but you can’t make me successful. I went to the board without studying then pride took over. I did well and passed. The Lesson I learned was that my senior NCOs knew what was best for me when I did not. Yes they got a feather in their cap but we all do when a Soldier that works with us does well.