That is strange because in AC19 (US) it is "Area per Person in sq. ft". Perhaps it is a typo so it is meant to be "Area per Person in sq. m.". You would think the script is nearly identical except for the units.
If you are using the AUS version Archicad 19, the zone tool 'Zone_Stamp_1 19' has the Occupancy field "Area per Person in sq. meter", and I can enter decimal values with no trouble at all. I use this for child care centre calcs.