18.04 can't upgrade to 20.04

[Liu]

showed notificaiton bellow:
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.

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'.

and I checked the ppa is OK.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.39
ProcVersionSignature: Ubuntu 5.4.0-49.53~18.04.1-generic 5.4.65
Uname: Linux 5.4.0-49-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 9 15:39:52 2020
InstallationDate: Installed on 2019-12-26 (287 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-09 (0 days ago)