Whenever I edit an attendance session for which I am using a QR code to allow students to record their attendance I get this error when I select save changes. the only thing I have changes is above this section of the page where I set the Automatic Marking to be "Set unmarked at end of session". Is this something I'm doing wrong while editing. Why would this field be set to zero? Be aware that I am only a teacher with no access to anything other than my own course, so suggesting I adjust a setting as admin is not possible.
You need to talk with the moodle admin on your site. They have changed the default settings for the attendance which is causing this.