Note that you have to have had a Norton 360 subscription with automatic renewal enabled and have had at least one automatic renewal processed before you are eligible to enrol in the complementary program.
I have had Norton 360 for many years and it has always been on automatic renew. My Norton subscription list shows the Screen Care as active with a renewal date in January 2025.
If you are using a VPN or ad blocking browser extension, try disabling both when accessing the registration page.
If that is not the issue, sounds like time to contact Norton Support to ask them to help. Contact Norton™Support - Norton Phone Number, and Chat Support . If using chat, when you can enter information, enter Agent Please to get connected with a real human.