How to troubleshoot the second processor which is not detected?

Only when using 2 CPUs, the second one isn't detected.

Error Code 8549 initially points to a major event, specifically identifying CPU1_DIMM_D1 as disabled. This event is indicated in the System Firmware Progress (Formerly POST Error) – Next Steps section of the corresponding System Event Log Troubleshooting Guide. Therefore, remove the disabled DIMM.

Try these other steps if either reseating or replacing the DIMM doesn't help:

1. Ensure the BIOS/firmware is up to date by going to the Download Center.
2. To clear the CMOS, refer to the Steps to clear CMOS using the jumper method section in How to Clear CMOS to Reset BIOS Settings.
3. Load the BIOS default settings as follows:
   1. Restart the system into the BIOS by pressing F2 during POST.
   2. After you're inside the BIOS, press F9 to load the BIOS default settings.
   3. Reset any customized BIOS settings.
   4. Press F10 to save the changes and exit.

Troubleshooting Server Boot Issues (Non-Interactive Version) for Intel® Server Boards