If you have updated your Goodnotes app recently, you may be prompted for an access code.
To resolve this issue, please restart your iPad by turning your iPad off and then on again. You may also try to force restart your iPad if needed. How to Force Restart your iPad
Please also try to close the app completely (https://support.apple.com/en-sg/guide/ipad/ipad79518d15/ipados) by opening App Switcher, swiping up, and then re-opening the app from the home screen again.
If the issue persists, please submit a ticket at: https://scgssict.freshdesk.com/support/tickets/new