You must restart your iMac in order to authorize Norton 360. Once Norton 360 installer completed installation I was alerted that Norton 360 has been successfully installed.
Signed back into my Norton account, downloaded & installed Norton 360 all over again. Restarted iMac again and this time Norton 360 was completely removed from the system. Finally I followed Apple's Mac OS Big Sur support instructions for removing software by simply going into my Applications folder, finding & placing the Norton AP into the Trash bin & emptying it. Oddly the non functioning Norton 360 Ap remained on the task bar after restart with no functionality as before. I tried uninstalling Norton 360 from my iMac using the Norton Mac removal tool & restarting system. I quickly realized it obviously had something to do with Apple's latest Mac OS Big Sur 11.5.1 security update. It would no longer open at all, instead a Norton 360 popup window appeared on screen t endlessly searching for itself & required a forced shutdown to close.
CURRENT VERSION OF MAC OS APPEL UPDATE
Once my iMac installed the latest Mac OS Big Sur 11.5.1 update Norton 360 ceased to function. Despite the Mac version of Norton 360 having only Basic functionality (ie.Virus Scanning but none of the many other Norton 360 Premium tools for Windows 10 PC's) it worked perfectly for almost 3 weeks. Had no issues installing Norton 360 on the iMac with Mac OS Big Sur 11.5.
CURRENT VERSION OF MAC OS APPEL INSTALL
Since Norton 360 Premium comes with 10 Licenses for PC's & Mac's and I only use 1 of the 10 figured I'd install Norton 360 on my new iMac as well. I already have a Norton 360 Premium subscription that I've used on my MSI GT76 Titan DT-039 4K Windows 10 Pro Gaming notebook for last 2 years that works flawlessly & comes with the full feature tool set of fully functional software such as Norton Backup, Norton Safe Search Browser protection, Performance monitor, etc. Purchased a new 2021 Apple iMac 27 Inch 5K Retina with Intel Core i9-3.6GHz 10 Core processor, 128GB DDR4 2667GHz RAM, AMD Radian Pro 5700XT 16GB GPU in early July 2021 that came with Mac OS Big Sur 11.5 factory installed.