This can happen if you have a question of type Constituent Registration Information (CRI) that does not include an email field. Without the email, when the survey tries to update the constituent record (as is demanded when you include a CRI question) it may not be able to find a record because it does not have a unique identifier. To fix this problem, either require users to be logged-in to submit the survey or require the email field as part of the CRI question.