Among the possible answers, I can't find the one I would say, what do I do? Is there a way to write my answer down?

The answers you can see are generated by the software according to the trend of the meeting and the way you managed the character's mood and answers. So it can happen that, like in real life, you won't find the exact answer you would have given.
Your goal is to choose the best possible answer among the 3 choices suggested in this particular step. 
You can't directly write the text of the answer/question you would have given/asked to the character.