I have seen custom themes hide that icon before - if you're using a custom theme can you try switching to "boost" to see if it re-appears? - also make sure you are using the latest version of the attendance plugin available for your version of Moodle - sometimes when an organisation upgrades their Moodle site they forget to update the plugins as well!
Attendance (plugin)
QR Code Missing
Αυτή η συζήτηση κλειδώθηκε κι έτσι δεν μπορείτε να απαντήσετε σε αυτή.