For UEFI-install your hard-drive should have GPT-partition-table.You need to create
ESP (EFI System Partition, 500MB, FAT32, boot-flag).
If your drive has msdos-partition-table you should convert to GPT first.
Or reinstall in legacy-mode.
Boot your Ubuntu-installer and choose
Something else. Open terminal with
Find out your partition-table-type:
sudo parted -l
Look for the entry
IF Partition Table is GPT then reinstall in UEFI-mode, create ESP during Install.
IF Partition Table is msdos then reinstall in legacy-mode or convert to GPT
and then reinstall in UEFI-mode.
How to convert msdos to GPT:
sudo gdisk sdX (for X type the correct letter for your drive)
Upon start, gdisk attempts to identify the partition type in use on the
disk. If it finds valid GPT data, gdisk will use it. If gdisk finds a valid
MBR (msdos), it will attempt to convert the MBR into GPT form. Upon exiting
with the 'w' option, gdisk replaces the MBR or disklabel with a GPT.
The MBR-to-GPT conversion will leave at least one gap in the partition
numbering if the original MBR used logical partitions. These gaps are
harmless, but you can eliminate them by using the 's' option, if you like.
How you boot is how you install. If you want to install in UEFI-mode, you have to boot the installer in UEFI-mode. If you want to install in legacy-mode, you have to boot the installer in legacy-mode. Just choose the correct boot-option when you start the installer.