A software update is required to connect to your iOS device / iPhone
The problem can be fixed by installing XCode beta.

This error occurs when the version of macOS (and iTunes) running on the computer is not compatible with the version of iOS on the device you're trying to connect.

Normally, updating the macOS to its current version will solve the problem. However, this won't work if the iOS device is running a newer beta version, and the Mac is not.

If you don't want to run the beta version of macOS (or downgrade the iOS device to a non-beta version), a solution is to install the beta version of Xcode.

Even though you may have no use for Xcode itself, its beta version includes the necessary software patches which will allow your existing macOS (and iTunes) to support devices running the iOS beta.

Download the Xcode beta from:
  https://developer.apple.com/download

The xip file will (eventually) unarchive into the full Xcode app. Drag it into your Applications folder, and then be sure to start Xcode. At that point it will install the necessary software components for the latest iOS beta. When it asks to open or create a project, you can quit Xcode.

iTunes should now recognize the iOS device.

Reference: https://ios.gadgethacks.com/how-to/fix-software-update-is-required-connect-your-iphone-warning-your-mac-0185898/