It seems you have a MBR/UEFI install which is not common (actually this is the first time I am seeing such a setup made by someone else) . This might be confusing the installer. Make sure you are booted in UEFI mode, manually partition the drive and try again.
looks a bit strange indeed… but seems ubuntu is running on uefi mode…
First is to make sure you boot installer ISO without any modifications some USB writing tools doing…
And second is to check if ISo is booted in UEFI mode:
in a terminal run:
For a BIOS mode install, you didn’t have to make an ESP (EFI System Partition) since the bootloader should be installed in the MBR of the disk and not onto a particular partition.
I guess, in order to install EnOS, you could boot your USB in Legacy mode, in the installer choose manuel installation and point it to the partition where you want it to be installed. Choose the MBR of the disk (/dev/sda) as the location for installation of the bootloader.
Is the computer UEFI? If it is then you should install it all in UEFI mode. Secure boot should be disabled, CSM mode turned off. No legacy UEFI only. Boot in UEFI mode not legacy. Install in UEFI mode. Don’t use legacy mode to boot the usb if your system is UEFI. If your computer does not have UEFI then install in MBR is obvious.
The OP has already installed Ubuntu in Bios/Legacy mode
So for a dualboot to work properly, that is, for the grub to be able to add boot entries for both systems to the boot menu, the second system also needs to be installed in the same mode.