That brings up the question, have you done anything trying to disable it?
I can not attest to Norton as I wouldn't touch that AV with a 10 foot pole, but normally the 3rd party AV registers itself during install. Also normally, once a 3rd Party AV registers with Defender your settings>security will reflect that another AV is managing your protection.
Question 1- Is Norton running in task manager?
Quetion 2 Is Windows Defender running in task manager?
If both are running, the only suggestion I have is to completely uninstall Norton using its removal tool, and restart. Verify that you can access Windows security settings correctly and nothing is grayed out. If anything is grayed out with Norton uninstalled, now is the time to troubleshoot that issue.
Reinstall Norton again to see if it will register correctly in windows security and automatically disable defender.