"I'm sorry; I have to refuse your invitation."
Translation:Je suis désolée, je dois refuser ton invitation.
Désolé and désolée are both equally correct. Duo always has one specific sentence with specific word forms that it is using as the correct answer to list for the exercise, and a host of others that are also correct and accepted. One thing I like about Duo is that it mixes it up and uses different forms for different exercises--it doesn't act like masculine is the default mode of being because, well, it isn't, is it? :)