do-release-upgrade from 18.04 to 20.04 - no development version
According to the Focal Fossa (20.04.1 LTS) Point-Release Status Tracking page, upgrades are not enabled yet due to "a few upgrade blockers".
This issue was reported as bug #1890936.
Upgrade from 18.04 is done with 20.04.1 (note the .1).
That point release is set for release on august 6th 2020. So you need to wait a little bit longer as it is not the 6th all over the world yet.
It was initially set for July 23rd but that got postponed to Aug 6th.