Ubuntu – EFI Partition vs /boot in UEFI System

You've booted in BIOS/CSM/legacy mode, so the installer is trying to set up a BIOS boot loader, not an EFI/UEFI boot loader.

I recommend you go back and find a way to boot in EFI/UEFI mode. One thing that might help with this is using an EFI-only boot medium. Unfortunately, the Ubuntu installation discs are Frankenstein-monster conglomerations of half a dozen more-or-less incompatible different systems that are made to work, after a fashion, through technical trickery. The trouble is that, although this sort of thing works much of the time, it fails sometimes, and when it does fail it becomes harder to disentangle everything.

The easiest thing to try is likely to be to create a second USB flash drive with my rEFInd boot manager. Insert both it and the Ubuntu boot medium in your computer and tell the computer to boot to rEFInd. If you're lucky, it will present a boot option for the Ubuntu installer and that will work. I can't promise that luck will be with you, though; you may need to go digging through firmware boot options or take some other radical steps to get it to work.

Oh, and you may also want to read my page on doing EFI-mode installs. It doesn't directly address the problem you're having, but it covers a lot of potential EFI installation and boot problems.

If your motherboard fully supports UEFI mode, enable that. Then, make sure when selecting your flash drive, you boot into UEFI mode. To do this, I recommend using dd. To find your flash drive

sudo fdisk -l

Let's say it responds with

/dev/sda
/dev/sda1
250 gb
/dev/sdb
/dev/sdb1
14.9 gb

Then you would

dd bs=4M if=/path/to/Ubuntu.iso of=/dev/sdb

To further clarify,
bs=4m Is the base size of four megabytes, a number that I found to be fast and stable with my devices.
if=/path/to/Ubuntu.iso Input file, and then the path to the file. (Right click the file, click copy, and click the terminal and select 'Paste filenames'
of=/dev/sdb Output file (usb stick, should be discovered with sudo fdisk -l and selected based on size)

After successful boot in UEFI mode, install and make a ~100 mb partition and make it an 'EFI boot partition'. That should be it.