new iPhone 12 says I need to download iTunes12.10.10 on iMac

The iPhone 12 and iPhone 12 mini - Technical Specifications - Apple give the following:

System Requirements

Syncing to a Mac or PC requires:

• macOS Catalina 10.15 or later using the Finder
• macOS El Capitan 10.11.6 through macOS Mojave 10.14.6 using iTunes 12.8 or later
• Windows 7 or later using iTunes 12.10.10 or later (free download from itunes.com/download)

So it should work with iTunes 12.8.2.3 installed. That version may also need to download a patch to communicate with the newer device, which should happen automatically. The MobileDeviceUpdater tool normally does it if that hasn't happened for reasons. Reboot computer and device and try again. You could also try reinstalling 12.8.2.3 while the device is connected.

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For iOS 14 you need iTunes 12.8.2.3 or better running on OS X 10.11.6 or better. See How to upgrade to OS X El Capitan - Apple Support if required.

Assuming a suitable version of iTunes is in place then in Finder use the menu item Go > Go to Folder..., copy the following text, paste it into the dialog box and press enter:

/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/Current/Resources

Unlock your device and connect it to USB. Double click on MobileDeviceUpdater in the folder that has opened. It should notify you of a software update. Install it and iTunes should hopefully recognize your device.

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