I have a late 2015 iMac. Unable to update OS beyond Monterey 12.7.6.

Monterey 12.7.6 is the highest macOS that your Late 2015 27" iMac can run.

If you need to run a later or the latest macOS version, then you need a newer or new Mac.

If you do not want or care for the smaller 24" iMac, consider getting a Mac mini and a 27" or 32" external display.

for help choosing your next Mac, see > Mac - Which Mac is best for me? - Apple

Make absolutely sure you know your Mac model.

To get more information about your computer, choose:  (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in "About System Information [Profiler] on your Mac" - System Information User Guide for Mac - Apple Support

Also note what the newest macOS your computer can run on this page. Identify your iMac model - Apple Support

The macOS version you can upgrade to depends on the Mac model and the year it was released.

Click the Apple menu () at the top of your screen and choose About This Mac.

➡️ macOS 12 Monterey is compatible with the Macs listed here

➡️ macOS 13 Ventura is compatible with the Macs listed here

It's correct. Ventura and above aren't compatible with that model.

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You're welcome. 👍