After replacing the Systemboard, 6200 Invalid serial number while booting
Edit the serial number in BIOS, it is not possible to do in normal BIOS, we need to get into Editable BIOS mode.
After entering BIOS Setup by hitting the F10 key, press Ctrl+A to open additonal fields in Security>System IDs menu.
Enter your PC's serial number in Asset Tag Number and Chassis Serial Number fields.
Save changes there and again when you exit the BIOS.