Latest macOS for iMac
Can't back up iPhone with (Retina 5K, 27-inch, Late 2014) running Big Sur 11.7.2
Earlier Mac models
Can't back up iPhone with (Retina 5K, 27-inch, Late 2014) running Big Sur 11.7.2
Earlier Mac models
A late 2014 iMac can be updated to Big Sur 11.7.9 which was released on July 24, 2023. See: About the security content of macOS Big Sur 11.7.9 - Apple Support
and Update macOS on Mac - Apple Support
Latest macOS for iMac