This issue appears on those who have updated to the iOS 8.4 firmware, including iPhone 6 and 6 Plus.
So those who face the same problem, do not be worry, because there is a workaround you can use to deal with the said bug, just follow the full instructions offered by Jailbreak Square blog, at the end the iCloud Music Library can’t be enabled error will be fixed once and for all.
How to fix iCloud Music Library can't be enabled error on iOS 8.4
I tried the solution below more than one time, firstly I wasn't successful, but I keep trying it over and over till I was successfully fix the problem. The Apple's servers might be so busy to serve users for installing the iOS 8.4 and enable their iCloud Music Library to enjoy, that's why I tried several times to finally succeeded.
- Turn on your iPhone / iPad / iPod Touch then got to its Settings.
- Now tap the Music (Remember this is not in the iCloud Settings).
- Now turn the iCloud Music Library to On.
- Now select to merge or replace your existing music library. (Some were successfully selecting replace, other were successfully with merge, you can choose one that works for you).
- Done. Hopefully it works for you. If you still fail, then repeat the process until it work. Seriously repeat it.