Update could not be calculated

[Nachitzu]

When i try to upgrade from ubuntu 20.04 to 21.04 an error display, this says:
"
Reading status information ... Done

Calculating the changes

Calculating the changes

Update could not be calculated

There was an unsolvable problem calculating the update.

If none of this is applicable, then report this error using
the command "ubuntu-bug ubuntu-release-upgrader-core" in a terminal.
If you want to investigate the problem yourself, the logs in
"/ Var / log / dist-upgrade" contains update details; on
In particular, see "main.log" and "apt.log".
"

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubuntu-release-upgrader-core 1:20.04.36 [origin: unknown]
ProcVersionSignature: Ubuntu 5.11.0-37.41~20.04.2-generic 5.11.22
Uname: Linux 5.11.0-37-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.20
Architecture: amd64
CasperMD5CheckResult: skip
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sat Oct 16 21:20:06 2021
InstallationDate: Installed on 2021-02-28 (230 days ago)
InstallationMedia: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to focal on 2021-10-17 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2021-10-15T19:30:27.849056