Can't upgrade Ubuntu 18.04 to Ubuntu 18.10

[Balakrishna]

The error reports :
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.31
ProcVersionSignature: Ubuntu 4.15.0-45.48-generic 4.15.18
Uname: Linux 4.15.0-45-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: XFCE
Date: Fri Apr 12 09:58:20 2019
InstallationDate: Installed on 2018-08-17 (238 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2019-04-12 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2019-04-11T21:16:31.638790

[Brian Murray (brian-murray)]

Did you try using ppa-purge as suggested? Its the oibaf graphics drivers PPA which is preventing you from being able to upgrade.

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]