Mackbook Pro 16" Mid 2015 16GB 1TB Flash Storage Stuck on Sierra

Hi Everyone -

I have never upgraded my Mackbook Pro 16" Mid 2015 16GB 1TB from Sierra to a more recent OS version. I have 342GB of available flash storage.

I want to update it to the latest that will load on my MacBook but nothing is showing up when I click on 'About this Mac" and then click on "System Updates."

It doesn't show any available system updates but it does list updates available for apps.

Any ideas or help?

Thank you in advance for your help.

Greg