Hi,
As I understand it's not possible for guests to take a quiz in Moodle. The only way to allow guests to log in is to create a new user and give guest login details.
https://docs.moodle.org/29/en/Quiz_FAQ#Workarounds_for_allowing_guests_to_see_a_quiz
Is it possible to reset this account for a new user, otherwise the next visitor will see the result from the last user. It could be confusing if they enter in the middle of an attempt.