Not able to upgrade from 16.04 lts to 18.04 lts [duplicate]
Solution 1:
Upgrades from LTS releases are scheduled after the first patch - 18.04.1 which is usually a month after the first release. THey need to make sure everything goes smoothly and then the software manager will allow it.
If you're anxious, which based on my experience you shouldn't be cause there are probably tons of bugs, you can still force that upgrade, but that's not recommended.
sudo do-release-upgrade -d
What I would recommend is a fresh (format) install, since anytime you upgrade you'll face incompatibility issues.