error when tried to update to ubuntu 20.04 from 18.04.

[saptarshi dasgupta]

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.

Got this message while attempting a upgrade from 18.04 to 20.04.
I removed all PPA's except teams and brave from Softwate and Updates GUI
Still getting same error

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-51.56~18.04.1-generic 5.4.65
Uname: Linux 5.4.0-51-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Oct 15 19:51:36 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-15 (0 days ago)
VarLogDistupgradeTermlog: