how to transfer Mp3 files from i tunes library to iphone 6

Hi there lSun Dancel,

It sounds like you have MP3 files that you cannot sync to your iPhone 6. I would first check to see if you have Apple Music and iCloud Music Library enabled on your phone, which would prevent syncing your music locally with iTunes. Navigate to Settings > Music to check this setting. If it is enabled then Apple Music may have been disabled on your computer, preventing the library from updating from your computer library to your phone and you should re enable it:

Check your Apple Music library settings

Mac: From the menu bar at the top of your computer screen, choose iTunes > Preferences and turn on iCloud Music Library.
Windows: Press Control + B on your keyboard to open the iTunes menu bar. Choose iTunes > Preferences, and turn on iCloud Music Library.

*When you choose Merge, songs on your device that are in the Apple Music catalog become instantly available to access on your other devices. On iPhone, iPad, and iPod touch, songs that we can’t match are shown on your other devices. But you won’t be able to play them on your other devices until you upload them from your Mac or PC with iTunes.

If it is enabled on both devices but not updating correctly, you can manually update iCloud Music Library by going to File > Library > Update iCloud Music Library. When that finishes then quit and relaunch the Music app to let it refresh the music list:

Force an app to close in iOS

1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.

If you do not have that enabled on either device and do not use it, then I would quit the Music and the Settings apps on the phone with the above article, then restart the phone as well as the computer and try again:

iOS: Turning off and on (restarting) and resetting

Let me know how that goes! Thank you for using Apple Support Communities.