Have you recently upgraded your hardware to the new Skylake system, only to find yourself with a machine that won't boot to install your operating system?
Even if you select the correct drive to boot from, say a USB stick, it will not boot from it.
Quick tip, that may solve the problem:
Go to bios, Security, and set an Administrator and User password.
Select the drive you want to boot from, and this time it should work.
When you are done installing the OS, you can disable the Administrator and User password again.