How do you write a letter to the boss about not attending training program?

You would want to be very polite, especially if the boss expects you to attend. Begin by apologizing for being unable to attend, and then give the boss good reasons why you cannot be there. Conclude by offering to make arrangements to attend at another time (you do not want the boss to think you are avoiding the training program; even if you think it's a waste of time, the boss wants you to be there, so you need to show a positive attitude about it). The boss may still be upset with you, but if you explain your situation and offer to attend at another time, it may not cause a problem for you at your job.