I have used Norton 360 for years on my macair now all of a sudden I cannot open it. It tries to open i have reinstalled but still won

There are no known Viruses in the wild that self replicate and affect macOS. There are Malware and Adware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application. For that purpose - Malware / Adware use Malwarebytes for Mac

One of the first mistakes people make is presuming self importance and that the Internet is targeting them for be robbed, deceived or otherwise specifically targeted IMHO

Secondly - relying on some Third Party Software as the first line of defence is truly a mistake. Due diligence when surfing the internet and avoiding un-secure site >> like http versus https << especially when purchasing or giving personal information for purchase or subscription to a service and / or general surfing

As outlined earlier - there are no versus that self replicate and affect macOS. Any software Company that touts this will protect your computer is bogus - period.

A very good User Tip prepared by a very Senior ASC Contributor is here https://discussions.apple.com/docs/DOC-8841

Yes, there is Adware and Malware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and gets installed along with the Application. For that very purpose, suggest downloading from a Trusted Developer and Respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

Also - only download and install Third Party applications Directly from Reputable Developers and from the Apple Apps Store.

 Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

Lastly, are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Am sure More Senior ASC Contributor may have additional insights and welcome them to make less the apparent concern for Safety.

It is good to be cautious and aware of what is happening on the Internet but . . a realistic perspective will serve one well.

Consider it a blessing.

Remove it. You don't need it, you don't want it.

It is completely useless on a Mac.

I concur with KiltedTim and Norton is the worst.

Welcome and safe computing.