Gui shows up for one player at a time instead of all players at the SAME time? [Solved]

It's because of the wait in the loop inside the function.

I recommend simply using spawn inside the message function, like so:

function Message()
    for i,v in pairs(game.Players:GetPlayers()) do
        spawn(function()
            --I changed this part to msg instead of message. You might not want this.
            local msg = v:WaitForChild("PlayerGui").ScreenGui.Message
            msg.Visible = true
            wait(3)
            msg.Visible = false
        end)
    end
end

while true do
    wait(20)
    Message()
    --Code to run
end

EDIT:

Scripts running on the server can't access GUIs from the players' playerGui that they did not create and add there, meaning it won't see the ScreenGui, or the Message.

You'll have to use Remote Events.

--// Regular Script

local remoteEvent = game.ReplicatedStorage.REMOTEEVENTNAME

while true do
    wait(20)
    remoteEvent:FireAllClients()
end

--// Local Script on the client; like in PlayerGui.

local remoteEvent = game.ReplicatedStorage.REMOTEEVENTNAME
local gui = DEFINE GUI

remoteEvent.OnClientEvent:Connect(function()
    gui.Visible = true
    wait(3)
    gui.Visible = false
end)