Show Memories Notification in photos on Mac won't turn off

Have you tried to restart the mac and and then tried again to disable the Memory Notifications?

If that fails, try to disable the notifications for Photos completely in the System Preferences > Notifications.

If the normal restart doesn't fix the problem try booting into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.