Can't Install Required Software Update
I have an iMac that is old enough to be maxed out at Big Sur. I can connect two iPhones to it via USB to back them up. But when I try to connect either of two iPads, I'm told that, "A software update is required". When I try to install the update, the iMac downloads it, but then the result is, "Can't install the Software." While all this is going on the iPad appears and disappears in the sidebar of a Finder window. As a result, I can't back up the iPads to the iMac.
The iPhones and iPads are all running iOS 16 and iPadOS 16. How can I resolve this?
iPad Pro, 12.9-inch, 6th Gen, Wi-Fi