When you say it doesn't boot into Ubuntu server, do you mean you can't get the installed system to run, or do you mean you can't get the installer to run? I wrote the instructions you are following so it would really help if you could be a bit more descriptive in what you've done and what you are seeing on the screen.
On pi3b+, booting from USB, I am able to run Ubuntu server iso arm64 and make arm64 installation.
I pre format my target usb as msdos part 1 fat32 and part 2 ext4.
grub install fails near the end (as seems to be expected) but everything else seems to complete ok.
After install part 1 is still empty and part 2 contains what appears to be good filesystem.
fstab looks good with mounts for / and /boot/firmware by UUID.
However per the other previous post I cannot set this up to boot from the fat32 part1..
I have tried a very large number of creative ideas for the part 1 fat32 including every post I have found on the subject many of which are divergent or contradictory. config.txt, cmdline.txt, cloning images from /boot into the fat32 partition 1, compiled uboots etc.
The wiki.ubuntu.com page is helpful but also confusing as it covers in the same document merged instructions for many diferent dated ubuntus, many different dated pis and many different arch and builds so it is difficult to know what is the magic combination for bionic/usb/pi3b+.
I believe that the kit and setup is good and and along the way I have been able to see it setup the new lan ok, request dhcp, receive and set the correct ip, try and retry to boot from tftp but I do not want to boot from the net.
I need a stand-alone usb bootable configuration - My knowledge of grub is not especially good.
Is there a "known to work" step by step for Ubuntu server 18.04.1 to allow boot (pi3b+/usb/arm64) of an installed system from part 2?