Can't update Lightroom Classic for Mac ?

It should be possible to install Lightroom Classic 15 if your computer is running a supported operating system version. Lightroom Classic 15 requires macOS Sonoma (version 14.x) or later, so see if your iMac is running or can install a supported version of macOS (currently macOS 14, 15 ,or 26).

If your iMac is too old to run macOS 14, then you need to understand that what happened is entirely predictable and known, not a surprise. Because there is a pattern that Adobe, Apple, and others have established for many years. They end support for older computers because with new models being released every year, as more years pass the total number of models to support grows too large to maintain and it becomes more difficult to make new features work on older, less capable hardware and operating systems.

These are the factors that determine when you must budget to buy a new Mac:

Apple announced some years ago that they provide updates for only the last three versions of macOS. In addition, Apple tends to provide about 7 to 8 years of macOS upgrade support from the date a Mac is released. After that, no more macOS upgrades or updates, which will lessen the ability to upgrade apps (see below).

Adobe announced several years ago that they support only the last two major versions of Creative Cloud software. For example, because Lightroom Classic 15 was released yesterday, the only versions available today in the Creative Cloud desktop app are 14 and 13. On the Mac, Adobe matches Apple support: Current Adobe apps support only the last three major versions of macOS.

Every year around now, both Apple and Adobe announce another major upgrade. If you combine that with the limited support periods, how long your Mac is supported becomes very predictable: Every year the Apple and Adobe system requirements ratchet up one major version, and every year the oldest supported version is dropped. So the closer your Mac is to about 7 years or so from date of release (not purchase), the higher the chance that it won’t be supported for the next year’s major Apple/Adobe releases. The year Apple releases a macOS upgrade your Mac can’t run, you know the clock is ticking: Within three years, the last macOS version supporting that Mac will lose update support from both Apple and Adobe.

If you put all that together, this is what you find: If you always want to run current software, plan and budget so that funds are available to replace the Mac by the time it loses all support for Apple and Adobe software updates. You can also choose to not replace the Mac to save money, but by doing so, understand that you must continue using your current versions because they can’t be upgraded.

There is an additional challenge with your iMac model because it has an Intel processor. The new Apple Silicon processors are so advanced and so much faster that Apple and Mac developers are either dropping support for Intel Macs sooner than usual, or, features that depend on the power of Apple Silicon might not be able to run well on Intel so they aren’t adapted for Intel.

Because your iMac has an Intel i7 processor, Apple might have already cut it off from macOS upgrades. The system requirements for macOS 26 Tahoe list only one supported Intel 27" iMac model, the 2020 model. And just about every Mac expert believes that next year’s macOS upgrade will not support any Intel Macs at all.