Dear John,
I am extremely sorry to tell you that I am unable to go to your party this Sunday.
I'd love to your party,but I have to prepare for my test at the end of this term and I have the flu,so I have to go to the doctor this weekend. So,I can't go to your friends party.Can you ask to me another time?If so,next Sunday I'm available,you can ask me that day.
I'm looking foward to your reply.
Yours,
Name