Was the MAS disconnected during the repairs while the key was "ON?" If so, erase the code and see if it returns. Usually if you mess up the idle air volume relearn, you will get a P0505 code, not a P0100 code. There was a TSB for this code on this model year Maxima. The bulletin stated that the code "MAY" also include loss of power, stalling and surging (but not necessarily). The repair included a new style mass air flow sensor, installing a genuine Nissan air filter and a new ECM software program designed for the new style MAS. You can access a copy of bulletin NTB01-036a here:
https://x.nissanhelp.com/forums/loca...p=10&keyid=467
Also, make sure there are no unmetered air leaks between the MAS and the throttle body (split air ducts or loose clamps on the air intake, for example) or leaking vacuum hoses. If that looks good, you may want to contact the ECM rebuilder. Perhaps another part of the board got damaged? They may want you to send it back so they can re-check it.