Strange. It looks like in your code the character 'ca' is not defined.
In your "script.rpy" file you should have the following line which define such a character:
Code:
default ca = Character("???", who_color="#7c23b0")#Carol
Did you make any changes to the original code?
Consider redownloading it.
Also, since the full traceback begins as follows:
it seems like you were in the middle of the Jessica Goodbye Story when the error occurred... I mean: this is towards the end of the game, and if I remember correctly it is not the first time a dialogue of the "ca" character is displayed, so it's very strange that this error did not show up before.
It seems like you edited the source code and reloaded the scripts after modifying it in the middle of the game: can it be?