What is Error Code 4000?

I solved the error code problem by uninstalling ITunes then downloading and reinstalling. The version on my computer was 13.?? The latest updated version. The version that I installed was 12.?? It worked straight away for the iPhone 7 but took 2 attempts for the iPhone X.

This was after hard resets of the phones and a factory reset of the iPhone 7 (which didn’t help)

Nothing seems to work. Have reset phones i.e forced reboot, reset to factory settings, changed cables, reloaded updates from iTunes, tried different networks etc. The iPads that we have updated correctly using the same cables, network and computer.