I'm rebuilding old computers for test equipment at my company because they are too cheap to purchase a cheap computer. It's costed more in my labor alone so far than a new cheap computer would cost. After installing the HD and booting up the machine, I get the error HDD failure. I've tried different machines and different HDD's what am I doing wrong?

Is the hard drive an IDE drive or SATA? if IDE, mess with jumper and/or put it in a different IDE header. If it's SATA try plugging it into a different SATA header. You've tried multiple drives and they all fail? Your hardware might just be dying (it's old, I'm assuming.)

It might be BIOS setting, set to work with original HD (one that used to be attached to the mobo). Try changing the setting to "automatic" or "auto-detect" for each IDE channel (usually, done by loading default BIOS settings), or use the HD detection utility in BIOS if available.
If you load default settings, expect that all accelerating features regarding HD and data transfer to be disabled. You might want to enable them.