When trying to restore iPhone/iPad/iPod via recovery mode, the popup error message below shows on iTunes:
The iPhone (iPad/iPod) cannot be synced or Problem downloading the software for the iPhone (iPad/iPod). An unknown error occurred (-39)
Solution 1: Disable Windows Virus & Threat Protection
Step 1: Press Windows + I button at the same time to open Windows Settings.
Step 2: Click Update & Security > Windows Security > Virus & Threat Protection > Manage Settings.
Step 3: Toggle off Real-time Protection.
Step 4: Select Yes on any prompts to successfully block the Antivirus from interfering during the backup process.
Solution 2: Update iTunes
Download the latest version of iTunes for Windows or Mac at apple.com/itunes/download/
Step 1: Open iTunes on Mac.
Step 2: From the menu bar located at the top of the screen, choose iTunes and then scroll down and click Check for Updates.
Step 3: Follow the instructions to install the latest iTunes version.
Step 1: Open iTunes.
Step 2: From the top menu of the iTunes window, choose Help and find Check for Updates.
Step 3: Follow the process of how to install the latest iTunes version.
Solution 3: Update Operating System
Open the App Store on Mac OS X devices, and then switch to Updates in the toolbar. If updates are available, click the Update buttons to download and install them.
Go to Start button > All Programs > Windows Update > Advanced Options > Select Automatic (Recommended) under Choose how updates are installed.
Solution 4: Remove Photo Caches
Find or remove the Photo Cache folders on Mac
If you sync from a Photos, iPhoto, or Aperture library, find the Photo Cache folder in its default location
Step 1: In the Finder menu, choose Go > Home.
Step 2: In the Pictures folder, find the Photos Library, iPhoto Library, or Aperture Library file.
Step 3: Right-click the file, then choose Show Package Contents.
Step 4: To remove the iPod/iPhone Photo Cache or Apple TV Photo Cache folder, drag it to the Trash.
Step 5: After deleting all photo caches then connect iPhone/iPad to Mac and open iTunes.
Step 6: Click the Sync icon from the iTunes interface.
Find or remove the Photo Cache folders on Windows
Search hard drive to find or remove the iPod or the Apple TV Photo Cache folder
Step 1: Click the magnifying glass icon at the Taskbar.
Step 2: Type iPod/iPhone Photo Cache or Apple TV Photo Cache, then Enter.
Step 3: Drag the folder to the trash.
Step 4: Restart iTunes.