Using _G but not working?

The reason it's causing an error is because you told the script to check the _G.Admin table in the position of "player.Name". Basically, the table is looking for a key which doesn't exist.

What you should do is do a for loop through the table, and see if the child is an admin, like so:

local isAdmin = _G.Admins

function findPlayer(name)
    for _, player in ipairs(game.Players:GetPlayers()) do
        if player.Name:lower() == name:lower() then
            return player
        end
    end
end

function onChatted(message, player)
if message == "Close" then
    for i, v in pairs (isAdmin) do
        if v == player.Name then -- You really should use the IDs like everyone said..
            game.Lighting.Door.Parent = game.Workspace
            break 
        end
    end
end 
--copy everything from 'if message == ":norma;"' until the first 'end' and paste it between this and the 'end' below this.
end


 --Copy and paste everything from if message == until the first 'end'  and change the building name and command each time.

game.Players.PlayerAdded:connect(function(player)
    player.Chatted:connect(function(message) onChatted(message, player) end)
end)

And you would do the same for the other script, though once again I'm not sure why you don't just put them in the same script. Hope this helped!

EDIT: Also, in your first one, those brackets are not necessary:

_G.Admins =
{
"conman0552",
}

Should work fine.

EDIT 2: Whenever you see this.. where is your global table? You need one LocalScript copy and one Script copy of it. Also,

function findPlayer(name)
    for _, player in ipairs(game.Players:GetPlayers()) do
        if player.Name:lower() == name:lower() then
            return player
        end
    end
end

function onChatted(message, player)
    if message == "Close" then
        for i, v in pairs (_G.Admins) do
            if v == player.Name then -- You really should use the IDs like everyone said..
                game.Lighting.Door.Parent = game.Workspace
                break 
            end
        end
    elseif message == "Open" then
        for i, v in pairs (_G.Admins) do
            if v == player.Name then
                game.Workspace.Door.Parent = game.Lighting
                break 
            end
        end
    end 
end




game.Players.PlayerAdded:connect(function(player)
    player.Chatted:connect(function(message) onChatted(message, player) end)
end)

You have a syntax error. You use [] for dictionaries (tables where each index has a value set to it). Example:

local Dictionary = {
    ["A"] = 1,
    ["B"] = 2,
    ["C"] = 3
}
print(Dictionary["B"]) --> 2

For more information on Dictionaries, read this.

For your admin list, you'll have to use a normal table. This means just use string values with names in them.

_G.Admins =
{
    "conman0552"
}