i am unable to upgrade from 16.04 to 19 .i get the below error "cannot calculate upgrade"despite using ppa purge

[Bones]

This is the error I get despite using ppa purge (removed deadsnakes and graphics-drivers)

)
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.

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.If you want to investigate this yourself the log files in '/var/log/dist-upgrade' will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.15.0-50.54~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-50-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sat May 25 09:48:24 2019
InstallationDate: Installed on 2018-03-15 (435 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-05-25 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2019-05-25T05:07:28.614692

[lotuspsychje (lotuspsychje)]

Please edit your bug title to the right version you are updating to.
perhaps also add your sources.list or remove all your external ppa's
with ppapurge