Can't connect my new iPad Pro iOS 12.1.1 to my old mac mini macOS 10.11.6
I have an iPad Pro running current iOS 12.1.1 , which I keep synced/backed up on my old mac mini which acts as household backup server, always on with big disks attached. The mac is running El Capitan 10.11.6 and can't run any newer OS.
Each time I connect the iPad, a window pops up on the mac prompting a software update, per https://support.apple.com/en-us/HT208831 , but the update fails.
When I click "Install" the popup is replaced by a "Downloading" window which starts with progress bar already showing complete, and hangs out for several minutes doing nothing. My network stats show no downloading is happening during this time. After some time, an "installing" dialog comes up showing no progress, and a 2nd "Downloading" window opens. After some more time both popups go away and a failure dialog appears saying "Installation Failed" "Can't install the software."
I have tried reinstalling iTunes 12.8 per https://support.apple.com/en-us/HT204095 but no change. I have no 3rd party security software installed.
During this time, the console app shows logs:
1/10/19 2:46:04.327 PM MobileDeviceUpdater[1627]: Using the default predicate.
1/10/19 2:46:04.327 PM MobileDeviceUpdater[1627]: Predicate for SU query: ANY tags CONTAINS[c] 'DEVICESUPPORT' AND 'board-id:0x%.2X:chip-id:0x%X' IN tags
1/10/19 2:46:04.330 PM MobileDeviceUpdater[1627]: Entered:_AMMuxedVersion2DeviceConnected, mux-device:2
1/10/19 2:46:04.337 PM MobileDeviceUpdater[1627]: Received an XPC event indicating device attach: Muxed iPad was attached
1/10/19 2:46:04.339 PM MobileDeviceUpdater[1627]: tid:230b - unable to query device capabilities
1/10/19 2:46:04.360 PM MobileDeviceUpdater[1627]: Discovered a device. Asking if the device is restorable.
1/10/19 2:46:04.391 PM MobileDeviceUpdater[1627]: MobileDevice reports the device needs an update. Asking the user to update.
1/10/19 2:46:04.391 PM MobileDeviceUpdater[1627]: Board ID 18/ChipID 32769 (named Beast) needs an update
1/10/19 2:47:18.792 PM MobileDeviceUpdater[1627]: Software update began.
1/10/19 2:48:08.771 PM MobileDeviceUpdater[1627]: Got an error during the update?! Error Domain=SUSoftwareUpdateErrorDomain Code=108 "Can't install the software." UserInfo={NSLocalizedDescription=Can't install the software., NSUnderlyingError=0x7fd9b25354e0 {Error Domain=SUErrorDomain Code=301 "The update has been deleted since being downloaded from the Apple Software Update server." UserInfo={SUErrorUpdateTitle=Mobile Device, SUErrorUpdateProductKey=041-14491, NSLocalizedDescription=The update has been deleted since being downloaded from the Apple Software Update server.}}}
Help?
iPad Pro 11-inch Wi-Fi, Cellular