upgrading from 14.04 to 15.04 is failed why?

[Md.Al-Helal]

i try to upgrade to ubuntu 15.04 from 14.04 LTS.when upgrading shows error ""Can't install 'ubuntu-desktop'

It was impossible to install a required package. Please report this as a bug using 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.""

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.19.0-43.49~14.04.1-generic 3.19.8-ckt10
Uname: Linux 3.19.0-43-generic i686
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Thu Jan 14 18:16:33 2016
InstallationDate: Installed on 2015-12-18 (26 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta i386 (20150805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-14 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update.manager.release.upgrades: 2016-01-11T22:31:33.456673

[Brian Murray (brian-murray)]

Please install one of the following meta packages, probably ubuntu-desktop, for the upgrade to proceed further.

2016-01-14 18:16:00,264 DEBUG none of the '['ubuntu-desktop', 'kubuntu-desktop', 'xubuntu-core', 'xubuntu-desktop', 'ubuntustudio-desktop', 'mythbuntu-desktop', 'kubuntu-netbook', 'lubuntu-core', 'lubuntu-desktop', 'ubuntu-gnome-desktop']' meta-pkgs installed
2016-01-14 18:16:00,264 DEBUG guessing 'ubuntu-desktop' as missing meta-pkg
2016-01-14 18:16:00,945 ERROR failed to mark 'ubuntu-desktop' for install (E:Unable to correct problems, you have held broken packages.)