Newest compatible operating system for iMac (Retina 5K, 27-inch, Mid 2015)?
This iMac model is virtually identical to the late 2014 iMac with Retina 5K display, whose maximum OS is macOS Big Sur (version 11). Both models have the same identifier as iMac15,1. Somehow, Identify your iMac model - Apple Support claims newest compatible OS for this iMac model is macOS Monterey (version 12), yet it is not even listed at macOS Monterey is compatible with these computers. I am very certain the oldest iMac models to run Monterey officially are those that were introduced in late 2015.
Earlier Mac models