I just installed Norton Internet Security on a friend's brand new Windows 10 PC. I know this is the NS forum, but I have the same issue on my new W10 Surface with Norton Security.
I am getting the Windows Defender update failing issue on this new PC, too. I tried the suggested solutions from this forum (disable Fast Start, reboot, manually disable Defender before installing Norton, etc) with no fix.
I just had a frustrating chat with Norton Support; they kept repeating Norton is active and Defender disabled (which I already know)and said the Defender updates failing is not a known issue. I tried to explain that this is more an annoyance than anything.
I'm wondering if anyone else still has this issue, and if Norton is not working on it how can I escalate the issue?