I would need a call id to able to investigate better ,but from what I see from this screenshot , the success is not a variable here , the function is returning an error and it the response could be a string not a json and if its a json you will need to use the response variable inside the function to set it as variable
and also try to use the prompt transition it can be easier to use and in this case it will work fine
Or try reproduce it and share a call id so I can take a deeper look