Ubuntu 18.04 LTS server full disk encryption
According to the 18.04 server release notes on
https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes#Ubuntu_Server
you will need to use the alternate installer to configure LVM (also RAID, multipath, vlans, bonds, or the ability to re-using existing partitions) on installation. You can download it from:
http://cdimage.ubuntu.com/releases/18.04/release/