Everytime I try to boot my PC (Dell optilex 755), this message will always appear and I can't install OS. My PC is broken and I tried to install OS from USB. I've used the usb to install OS on other PC too. It works fine. If it's possible, I wanted response as ASAP because I have an assignment to make.
What operating system are you trying to install and how did you make the Bootable USB? I recommend making the Bootable USB baring in mind this is a Legacy System and thus the MBR partition scheme should be used for a Legacy BIOS.
Dr Philip Yip Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide