To check for any N codes in the register please go to Reporting > Reports > Attendance (tab) > Gaps in Register by Date:
In this report if you are greeted with the below message you will need to go to Admin > Config > Core Set Up > Holidays to ensure that none of your dates are overlapping:
For further information on removing the overlapping dates please click here.
Once in this report ensure that in the top right you have clicked to show
Any dates where N codes are present will be circled in blue, if you click into the circle you will then have the option to fix N codes.
If you click onto fix N codes this will take you to the manage register area where you can click into N code to add the required code.