I'm trying to allow the use of Expulsion Pylon for my world which is in a separate folder from the main one, going to the World directory, in serverconfig/pylons-server.toml I come across the following:

[expulsion_pylon]
#Which dimensions the Expulsion Pylon is allowed to operate in.
expulsionAllowedDimensions = ["minecraft:overworld"]

I'm sorry for the question but I really can't, I would like to know what I should put to allow my other world, the name is Void.

Run the command /forge tps to get a list of dimension IDs. It should give you a name in the same format, something like "mod_name:void". Then, add it to the list like this: ["minecraft: overworld", "mod_name:void"].