Since you're using a dictionary instead of a numeric table, you can't do buildingTypes[someNumber], you also can't use #buildingTypes

The operator # means number of items. You can use it to get the number of items in tables/strings, but you can't use it to get the number of items in a dictionary. If you do, it'll return nil. That's why the script is erroring, because you're trying to do math.random(1, nil)

There is a way of doing what you want to do though:

Approach One:

If you really wanted to get a random item in a dictionary, you could store the keys in a separate table and then get a random key from that table.

local buildingTypes = {
    ["Corporation"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Shop"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Food"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Utility"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}}
}

local keys = {}

--populate the keys table with the buildingType keys
for key in pairs(buildingTypes)do
    table.insert(keys, key)
end

--get a random key: 
local randomNumber = math.random(1, #buildingTypes)
local randomKey = keys[randomNumber]

--get building at random key: 
local randomBuilding = buildingTypes[randomKey]

Approach 2:

If we restructure the table a bit, we can have each building be a table instead. This way we can use math.random() to get a random position in the table:

added a variable called MAX_OCCUPANTS as an example, add/remove whatever you want.

local buildingTypes = {
    {Type = "Corporation", MAX_OCCUPANTS = 320},
    {Type = "Shop", MAX_OCCUPANTS = 420},
    {Type = "Food", MAX_OCCUPANTS = 100},
    {Type = "Utility", MAX_OCCUPANTS = 10 },
}


local randomPosition = math.random(1, #buildingTypes)
local randomBuilding = buildingTypes[randomPosition]

@royaltoe With some minor modifications, your system worked! Thank you! For anyone interested, here's the code:

llocal buildingTypes = {
    ["Corporation"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Shop"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Food"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}},
    ["Utility"] = {[1] = {"Some stuff here"}, [2] = {"Some stuff here"}}
}

local function randType()
    local keys = {}
    for i, v in pairs(buildingTypes) do
        keys[#keys + 1] = v
    end
    return keys[math.random(1, #keys)]
end

function buildings:ChooseName()
    local buildingType = randType()
    local prefixes = buildingType[1]
    local suffixes = buildingType[2]
    local prefix = tostring(prefixes[math.random(1, #prefixes)])
    local suffix = tostring(suffixes[math.random(1, #suffixes)])
    local name = string.format("%s %s", prefix, suffix)

    return(name)
end