Error -54

Follow the steps in this support article:

https://support.apple.com/en-us/HT205597

Good luck!

Okay - So the same thing happened to me recently after I signed up for the itunes music 3 month free subscription thing. I first followed all of the steps in the support article for error -54 but was still receiving the error - but you may want to follow those steps first just in case. So, I found another article elsewhere where someone stated turning off icloud music storage temporarily solved the issue for them - but in the newer updates that setting isn't there in the phone now. With it on, if you click your phone in itunes you'll not even see the option to sync all music. With this option turned off, suddenly the sync music option reappears when you plug your phone in.

So what I did is, in my iphone, Settings > Music > Sync Library > off and add playlist songs > off. I also went ahead and turned icloud drive off under icloud settings just in case.

Then I was able to plug in and re-sync my music. A pop up came up asking if I wanted to delete and resync my music and I said yes - this ended up creating a duplicate playlist but at least all the new purchased songs I added from my pc library are now there. Finally, the music has sync'd. Afterwards, I turned idrive back on and went back into the Music settings to turn sync Library and add playlist songs back on so my itunes music subscription streaming library and playlists would return to normal because turning sync library off will also make your itunes streaming playlists disappear temporarily - but somehow turning it off allowed the pc to sync the new songs to the phone. So turn off sync library in your phone, plug in and sync with itunes in pc, then turn sync library back on in phone.