>If you want just Ubuntu, then enter your computer's firmware and switch from UEFI to legacy. Then use the normal installation process.
>If you want dual boot and the current OS uses EFI (which is probably your case), then you can switch to legacy only temporarily to install Ubuntu. You must setup the partitions manually. At the end you'll have to switch back to EFI: >https://help.ubuntu.com/community/UEFI#Converting_Ubuntu_into_EFI_mode