Huh?
UEFI is Secure Boot functionality.
This enabled your system to run more than 1 OS. UEFI was implemented as a security measure, but can be manually disabled to allow other OS's to be installed (i.e. bootloader rewritten).
I just told you what to do: disable UEFI (or Secure Boot... different BIOS's call it one or the other usually) and you should be able to install another OS.
Carnage, How does one disable it plz if that is the case?