== Error -54 ==
From an OS9 reference, error -54 is a software lock on a file or a permissions error. (Some of the old codes still apply to OSX although there is no formal list of OSX error codes.)

First, try simply restarting your computer.

iTunes Library cannot be saved (Error -54) - https://discussions.apple.com/thread/1912814

Unknown error (-54) while syncing ipod - https://discussions.apple.com/thread/1082953 - problem was locked files

Why does iTunes keep showing a -54 error when I sync my iPad? - https://discussions.apple.com/thread/3727114

Error -54 possibly related to Touch Copy - https://discussions.apple.com/thread/3727114 - cleared by deleting preferences on iPad.

Restart solved my problem as well!

Restart sorted my issues too. 

Thanks  

OK this worked for me:

Reset the Lockdown folder in iTunes - Apple Support

I had this problem when I would try and sync wirelessly. Connecting the cable and syncing it that way solves my issue. I think it happened when I was trying to sync some PDF files from iBooks on my iPhone to my computer, it was the first time I had tried syncing since upgrading to El Capitan.

Restarting or connecting the iPhone with the cable didn't resolve my problem. However, although I get the -54 error message which says that the computer can't copy the phone, it still seems to back up OK and sync my selected music to it. I don't know if I should just ignore the message or if it matters.

I have this same problem (did the restart, reboot, Lockdown folder), it says that itunes can't copy my ipad to the NAS drive that contains my itunes content, yet it backs up the ipad fine (backs up on my computer) and syncs the content from itunes to the ipad fine.  Wierd.  Even after the latest ios update, no change. I am just ignoring with the hope it gets fixed.

Just found this, Itunes known error.

http://www.mcelhearn.com/itunes-12-3-not-copying-purchased-content-from-ios-devi ces/

iTunes 12.3 Not Copying Purchased Content from iOS Devices

If you download new content to an iOS device, or update apps, this content should be copied to your iTunes library when you sync the device. A number of people are reporting that iTunes 12.3 is no longer copying purchased content from their iOS devices. I am seeing the same thing on my iPhone and iPad, both running iOS 9.0.1.

This seems to be a bug, and I have filed a bug report with Apple. For now, I can’t find any workaround that will allow you to copy this content to an iTunes library.

Update: Apparently, this is the new behavior. Apple responded to someone who filed a bug report, saying:

This issue behaves as intended based on the following: Apps are no longer transferred from iOS 9 devices.

Apparently, this is related to App Thinning, a feature whereby iOS devices only download the resources they need to run apps on the specific device. To get the full app, you need to download it to iTunes.

This is a bit problematic. I have poor bandwidth, so I need to remember to no longer update apps on my devices, so I don’t have to download them four times.

However, I also tested this with music download from the iTunes Store, and it is not being copied either. So while the app sync issue may have a reason, the music sync does not.

Apple have no idea how to fix error 54!

it seems as if the problem is greatest on Windows 10

i myself have tried all options offered including changing folder attributes as well as files, no go!  iTunes folder still have the read only displayed.

so what about my statement?  Apple have published a feeble attempt and points out "The error can't be fixed by reinstalling iTunes or restoring your device to its factory settings" https://support.apple.com/en-nz/HT205597

personally I think this may be a windows 10 issue but it has been bouncing around for years and even on Mac... What's changed is the it is impossible to change the attributes on the iTunes folder at all.. I think iTunes can see this when syncing even though all the files are ok for read and write the folder is not and this is probably a Microsoft issue as Windows try and protect all folders it deem necessary to protect... Go figure.  And yes people I believe I have really tried everything

I also tried changing the permissions on my iTunes folder, but that did not work.  Mine seems to work now after I reset my iPad to factory status and then restored from backup.

Actually the lockdown reset only worked once. Error 54 came back with a vengeance and the lockdown reset doesn't work anymore.

I was inspired by “software lock or permission error“ and found a solution for my problem: I was alerted that three apps couldn't be download during an app update due to an -54 error. I browsed to the Downloads folder within the iTunes Media folder and deleted the temporary entries associated with these apps. Afterwards I could restart the downloads from within iTunes and it worked.

I had a similar issue with my iphone when connecting to computer/itunes. I tried most of what was listed on a lot of these related posts but nothing worked. finally i went into my settings-->music and realized that "icloud music library" was not turned on. I turned it on and tried syncing again and it went perfectly. maybe this will help someone.