Please refer to the following link:
How to Fix Unknown Error (-54) about iPhone/iPad/iPod Cannot Be Synced
Sometimes fixing your iTunes sync error (-54) seems a little difficult because it is hard to find which files cause this problem. In this case, you have to sync separately to find what kind of data you can't sync. For example, you can sync only songs to iPhone 6 or iPhone 7 to check whether the music files cause this unknown error (-54).
Of course, there are some common tips you can try to fix this error:
- Update your iTunes to latest version, now the latest version is iTunes 12.6.2.
- Upgrade iPhone/iPad/iPod touch to latest iOS operating system. Now you can update to iOS 10.3.3. (The iOS 11 will come out soon, you can update to it when it is available.)
- Authorize computer in iTunes.Open iTunes > Choose "Store" on iTunes menu and authorize your computer.
- Change permissions of your "iTunes" folder or original files/folder.
You can find the originals that you want to transfer to iOS device and change permission. iMore give us one example to change "Read Only" permission of "iTunes folder" to fix iTunes sync error (-54) on Window 10:
Open Windows Explorer > Go to "Music" folder > Right Click "iTunes" folder > Properties > Uncheck "Read Only".
- Run iTunes in administrative mode.
Sign in to Windows as an administrator > Right click iTunes icon > Select "Properties" > Select "Compatibility" > Check to run as administrator.
- (For Windows iTunes users) Install all available Windows update service packs. iTunes sync error may be caused by incompatible driver on your windows pc.
- Use an iTunes alternative
After trying solutions mentioned, if your iPhone still cannot be synced due to unknown error occurred (-54), you can try some iTunes alternative, like AnyTrans. As one of the best iTunes alternatives, AnyTrans allows you to transfer media files (like Music, Playlists, Movies, TV Shows, Podcasts, iTunes U, Audiobooks, Voice Memos, Apps, Ringtones, etc.), and won't erase the existing data on your iPhone iPad. Click Here to Download AnyTrans >