Upgrade to 20.04 from 19.10 isn't available [duplicate]
Solution 1:
I found this note in How To Upgrade Ubuntu To 20.04 LTS Focal Fossa - LinuxConfig.org
PLEASE NOTE
Upgrades from 19.10 will not be enabled until a few days after 20.04's release. Upgrades from 18.04 LTS will not be enabled until a few days after the 20.04.1 release expected in late July 2020. There are no offline upgrade options for Ubuntu Desktop and Ubuntu Server.
I have to wait some days. Hopefully works!
Solution 2:
If you want to upgrade early then you can use
sudo do-release-upgrade -d
on kubuntu or
update-manager -d
for a new development release.