When you try to sync your iPhone, iPad, or iPod touch with iTunes, you might see an alert message that says:
"The iPhone [device name] cannot be synced. An unknown error occurred (-54)."
This can happen if a file on your computer or iOS device is locked. In most cases, your sync will continue if you click OK at the bottom of the message.
If your content doesn't sync, restart your computer and your iOS device. Also, make sure that you have the latest version of iTunes and the latest software updates for your device. If you still see the issue, continue below.
Consolidate the media files in your iTunes library
The media files in your iTunes library might be stored in several locations. Try consolidating your files into the iTunes Media folder.
Check for issues with third-party security software
Some third-party software, including security software, can interfere with the sync process between your computer and your device. Change your security software's setup and see if it fixes the issue.
Sync a small amount of content
Sync only a small amount of content at a time. Add more content until you see the error again. By doing this, you'll identify the content that causes the issue.
Delete and re-download content
Delete and re-import content
If the content isn't from the iTunes Store, delete and then re-import it from the original source. For example, delete a song and then re-import it from the CD.
Remove PDF files from the sync process
This issue might happen when you try to transfer iTunes purchases from your iOS device to your computer. This might also happen when you try to sync with iBooks, because it uses the same process. If so, there might be a problem with a PDF file saved to the iBooks app on your iOS device.
To fix the issue, use the Share option in iBooks on your device to email the PDF to yourself. This saves a copy of the PDF in your email account and shares it with the email address you choose, which can be accessed from your computer. Then, remove the PDF file from your device.