serial number

Applications/Utilities/Terminal.app

From the terminal.app, copy and paste this command line:

 system_profiler SPHardwareDataType | awk '/Serial/ {print $NF}' | open -f

if the board is properly flashed it should return the serial number in plain text, you can copy it from there.

Take your Mac back in to the folks that repaired it, and ask them to set the serial number.

If you have not had the Mac under repair—the phrasing here is not entirely clear to me—contact Apple Support.