createCenter is used to create a side when no units of that side are intially on the map.
For example, if you put only west side units on the map, east side will not exist, and you will not be able to create any group of east side. But if you place a single east unit in the map, east side is created and then you can create any east group too.
If you want to spawn all your units using scripts, so you will start with none but the payer and its group, then use createCenter at init.sqs to be sure other sides exist. createCenter east, createCenter resistance, createCenter west or/and createCenter civilian.
May be you would also be able to createCenters (sides) for enemy and friendly. enemy and friendly are listed as sides, units in enemy side are enemies to everyone, units in friendly side are friendly to anyone.