Getting error -1426 when trying to transfer files from iPhone via iTunes File Sharing
I don't recall ever seeing that specific error code with iTunes before. 

However, when it comes to the other 14xx* error codes I've seen, the fix is usually doing one of the following:


Restart your Mac (or PC) and change the USB port you're using
Restart your Mac (or PC) and change the USB cable you're using


If neither of these works, reinstalling iTunes may resolve the issue.

Another option worth trying is to download and install iTunes 12.6.3 (this can be installed alongside newer versions of iTunes). 

Refer to Deploy apps in a business environment with iTunes on Apple's website for further information and download links.

Let me know if none of the above works.

* More specifically, error codes 1413, 1415, 1417, 1418 and 1428.