Why is this error called?

You have a couple thing wrong.

table.insert takes 2 arguments, the table and thing you want to insert.

example:

local Table = {}

table.insert(Table,"Hello")

The for loop you were using is not the best for tables, use a in pairs loop instead.

Here is a fixed version of your code:

local unions = {}

function DefineForUnion() -- 
local things = game.Workspace:GetChildren()
 	for i , v in pairs (things) do
		if v.className == "UnionOperation" then
			table.insert(unions,v)
		end
	end
end