Can't find PSU BIST button on Dell Optiplex 755

My Dell Optiplex 755 has been facing a problem recently whivh may be a power supply issue. When I try to turn on the PC, the light on the power button blinks amber.

 I've read a few articles about this and It may be due to issue in power supply. I've read that the Built-in power supply self test button is usually found near the power cable input socket or can be seen inside the computer chassis, I can't find it anywhere though.

 There is just the back side of a screw where the buttpn should be and some red thing that says 250V next to it.