Comment 83 for bug 1767703

There used to be a prompt that while worded poorly, did give you the ability to switch to a bios mode install even though you booted the installer in UEFI mode, but it was removed in 18.04. Unfortunately, instead of making it switch to a bios mode install by default, it keeps with the UEFI install and crashes. This mistake is being tracked in bug #1668148. Hopefully this will be fixed in 18.04.1, but until then, you just have to get your system to boot the installer in bios mode. If you are using a USB stick you can delete the EFI directory to make sure it can't boot in UEFI mode.