Couple of things to try.
1. Did you install the standoffs between the motherboard and case?
2. All power connections are in? If you miss your 8 pin EPS, you will not boot. Same to the 24 pin ATX.
3. Did you turn the power switch to on, on the PSU? That will cause a no boot.
4. Reseat RAM.
5. Reset BIOS. There is a pin for that. Look in the motherboard manual for that.
6. reseat CPU.
7. Pull board and make sure you only installed standoffs where they are needed. If you have more than is needed installed, it will short and not boot.
8. Paperclip test the Power Supply. Google it, it is easy to do.
9. Give up and take it to a shop.
I just though of another test too. Look at the wires for the front panel and make sure that they are attached right. I have done that wrong before. my reset button did not work for a week or more because I attached it backwards.
That is why it is not starting. JCOM1 is for a serial port. Those are ancient ports.
What motherboard do you have specifically?
They go in the JFP1 port using the pattern shown below.
MSI are officially arseholes. No Bebug LED.
Okay, Reseat RAM, and check that all power leads are attached.
No. The 4 pin is for P4 adapters. It is just like the floppy plug that is on your MOLEX lead.
As long as your EPS lead is plugged in, you are fine.
no. They go to IDE hard drives, Water pumps, Fans (when not attached to the motherboard) and several other add ons.
Reset the bios.
I have used 10 stand offs in total, I am not sure if that is the right amount?