Unable to upgrade 16.04 to Bionic Beaver 18.04? [duplicate]

Solution 1:

As stated in the "Upgrading from LTS" section of the Release Notes,

Upgrades from 16.04 LTS will not be enabled until a few days after 18.04.1's release, expected in late July.

The idea behind this is that people who upgrade from one long term stable (LTS) release to another will most likely appreciate the improved stability of 18.04.1.

Solution 2:

You need to use sudo do-release-upgrade -d prior to July 26, 2018 and drop the -d after that.

I highly recommend doing this on a test partition with a clone of your 16.04 system. See: Bash script to clone Ubuntu to new partition for testing 18.04 LTS upgrade