CORRECTION:

Title: Sorry, I am involved in a meeting that is irrevocable.

Contents: I am sorry, Cen.

REPLY:

Hi, Jinhyun! Thank you for this short message. I appreciate your thoughtful gesture.

I understand that you won't be able to make it tonight however I will surely miss you. ^_^

I'll just see you then next meeting.

Have a successful meeting and take care asyou go home.

Bye for now~~

your teacher,

Cen

P.S.

As for you above sentence, it is okay and acceptable to use 'is not changeable' in your sentence however the common term for it is 'irrevocable.

same