I'm assuming you want to remove the notification badge from the control panel, The below is supposed to reset the badge but doesn't look like it works to remove the 1. Using -a or -u and then a number increases the badge and then -r returns it back to 1.
/usr/syno/bin/synoappnotify -c SYNO.SDS.AdminCenter.Application -r @administrators
I have also tried using SYNO.SDS.AdminCenter.Update_Reset.Main as the class but same result
synoappnotify -c SYNO.SDS.AdminCenter.Update_Reset.Main -r @administrators