Mojave (and OS after that) cannot be installed on this computer

I'm am trying to update my iMac 2011 model to Mojave and it gives me the prompt "cannot be installed on this computer". I am currently operating with High Sierra 10.13.6. Is this as high in the OS's as I can go?

This OS has ruined quite a bit of my photos taken with the iPhone 6 at the time and I am hoping that by updating the system it will (allow optimal performance for the iMac) restore them as their original state.