I take it you checked for jumpers and without any extra parts. That is, one stick of RAM, no drives, etc.

As to jumpers, a funny error was the manual showed a HDD LED connection and it was actually a sleep jumper so half the time the machine power up, beep but nothing more. REDUCE THE MACHINE to just a few parts. You don't need more than the Power On connection and speaker, the mainboard, PSU, CPU+HSF and a stick of RAM. You don't even need a case.

Many builders start with all parts. Trouble follows.