Please refer following segment, Ubuntu MinimalCD only supports Legacy installation, it doesn't support UEFI installation,
When Customer needs installing Ubuntu mini system in UEFI mode, Please follow Ubuntu suggest to install Ubuntu Server amd64 iso file (64-bit) for 'mini installations' in UEFI mode
mini system in UEFI mode
While the minimal iso image is handy, it isn't useful for installing on UEFI-based systems that you want to run in UEFI mode. The mini iso lacks the proper files for booting the computer in UEFI mode. Thus, the computer will boot in BIOS compatibility mode, and the installation will be in BIOS mode.
You can use an Ubuntu Server amd64 iso file (64-bit) for 'mini installations' in UEFI mode. There is a compressed image file dd_text_16.04-UEFI-n-BIOS-4-pendrive-7.8GB.img.xz of such an installed system, that can be used as a start of a custom installation.
See this link: Installation/UEFI-and-BIOS/stable-alternative.