How to fix unknown error 10 on an old Mac?

See if this Apple article help: iOS update and restore errors

iOS update and restore errors – Apple Support (UK)

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Update the software on your Mac or PC

If you see error 10, your iOS device may have a newer version of iOS than is supported by your computer. You'll need to update the software on your Mac or iTunes on Windows before you can update or restore your iOS device.

macOS 10.11 and later:

1. Disconnect and then reconnect your iOS device to your computer. You may need to unlock your iOS device before it is recognised by your Mac.
2. When you see a message that says a software update is required to connect to your iOS device, click Install to download and install the software so your Mac can update or restore your iOS device.

Windows: Update to the latest version of iTunes.

If you continue to see error 10, or you don't see a dialogue asking you to update the software on your Mac, contact Apple Support.