I think this is similar to what others are posting. I am running Big Sur version 11.5.2. and I also keep getting the "You haven't finished Setup and your computer is not protected" but I do seem to be able to reinstall the product. The issue I have is that where previously I would click on the finish setup prompt and it would take me through the process of finishing the installation, now when i click on the prompt nothing happens. I can see that Norton is trying to launch and for an instant the green tick will appear and disappear but then nothing happens. If i then try and launch or run norton the same thing happens (a green tick will appear for a second and disappear, the app will have the dot underneath it to show it is running and them close down) even after i restart my machine. I have tried reinstalling norton but as soon as i restart my Mac the cycle repeats with the "You haven't finished Setup and your computer is not protected" and then norton stops running.
[Edit: Clarified subject]