"I am sick. I can't attend your party."
There wasn't an option for "unnatural chinese sentence" so i just wanted to mention here that this sounds like a very weird way to talk. If a non native speaker said it, sure fine. (It is understandable) If a native speaker talks like that i might check for a zip if he/she is wearing a human skin suit.
我生病了，不能参加你的派对了。 would be a step towards a more natural sounding sentence.
That 了 at the end sounds even more unnatural. Personally, i think the answer is fine as it is, but