The easiest, headache free way to get started is to (Using your Ubuntu 18.04 LTS Operating System) format your SD card with the FAT file system using the Disks application, and then use etcher to put the server image onto the SD card. Once etcher has the server image on the SD card, re-open the Disk...

My post on this forum may help you:
viewtopic.php?t=215724

No problem, glad I could help. You can Download my comprehensive guide for Ubuntu 18.04 Server on Raspberry Pi 3B+ from my Mega at:
https://mega.nz/#!bCRzAawR!w4E_R4R89eLZ ... BPMVoqvwLY

Booting the official Pi 2 image on the Pi 3B+ This Tutorial assumes the user will be using Ubuntu 18.04 LTS to prepare the SD card for the ubuntu-18.04.1-preinstalled-server-armhf+raspi2.img.xz and to configure its file system. You will need to have your Raspberry Pi connected via Ethernet to get st...