This generally happens whenever the survey submission page defined for the survey is no longer active. To see if this is the case:
  1. Content > Survey > "Edit" next to the survey in question
  2. On that first page scroll down to question 11, "Survey Submitted Page". Whatever you see there is where the survey is trying to take the constituent after they finish the survey. This can be any page but is likely a PageBuilder page. If so, copy the name of the page (will be at the end of the URL) and go to Content > PageBuilder and search for the page. Make sure that the page says status "Active", if not you may need to activate it. You also want to make sure that the security category of the page matches up with the security category for the survey (don't make the submission page more restrictive than the survey).
  3. To fix this you can either change the page defined in step 2, activate the page that is already there if it is not already, or clear that option. If you choose the latter, the standard survey submission confirmation page will be used.