Answer: This question does not have a specific answer, nor does it elaborate further into the question, so I will provide an answer that may or may not be approximately what the question is asking.
Dear [classmate’s name],
I would love to attend your birthday party, but unfortunately I won’t be able to. The reason for this is because [state the specific reason why you are unable to attend the birthday party]. Happy Birthday!
From your classmate,
[your signature]
