You may find a fresh installation of Citrix Hypervisor (previously XenServer) will not boot on relatively older systems. In my case a Lenovo ThinkCentre from 2011 threw the error:

This seems to be caused by the UEFI-enabled BIOS not supporting GUID Partition tables (GPT).

While it isn't necessary, I choose to force the BIOS' boot mode to Legacy:

Next we need to add the disable-gpt kernel command line argument when we run the installation.

If your BIOS supports UEFI booting when the Citrix Hypervisor installation media runs you'll be greeted with a fancy GRUB splash:

Enter menu.c32 at the prompt:

This will load the conventional menu you're probably accustomed to:

Press the E key to edit the boot arguments and add disable-gpt to the end of the linux kernel line as shown:

If you have already configured your BIOS to boot in legacy mode you will instead be greeted by a more minimalist boot menu:

Press the Tab key to edit boot options and append disable-gpt to the kernel command line:

Now continue to boot and complete the installation as normal.