Something is causing my GUI to not change the text to what I need it to, can you help me?

Message is deprecated, use Instance.new("TextLabel") instead if you're trying to get the messages to pop up on each user's local screen. If you're trying to make it appear as an object in the workspace for everyone to see at a static location, then use a BillboardGui/SurfaceGui with a TextLabel inside of it.

Delete the lines about parenting msg and make sure the TextLabel is visible when the text starts. This will create a floating message using a pre-built part/surfacegui/textlabel:

math.randomseed(tick()) --makes your math.random more random
--I suggest just pre-building the Part/BillboardGui/TextLabel
--You may need to fiddle around with the size and/or text alignment to get it to show correctly
local part = game.Workspace.Part --Premade Anchored part that has transparency set to 1 and is 10 studs in the air
local surfaceGui = part.SurfaceGui --Premade surfaceGui
local showingText = surfaceGui.TextLabel -- Premade TextLabel that has a background Transparency set to 1 and font size set to 50
local Players = game:GetService("Players") --Moved out here, no need to call it multiple times
local tickRecord = tick() --Records the time at the very start of the server
local counter = 0
local m

--The following table uses numbered keys to match the seconds in the game so we can conveniently use a counter to change the text
--It uses mixed keys, but that doesn't matter since we aren't iterating through the table
--Note that the GameTime is actually 200 seconds, the 35 seconds is for the wait time
local minigameText = {
    [0] = "Choosing Minigame.", [1] = "Choosing Minigame..", [2] = "Choosing Minigame...", [3] = "A Minigame has been chosen!", 
    [6] = "The Minigame is...", [9] = "Obby", [11] = "Get to the top before the water gets to the top", [16] = "The game will be starting in:",
    [17] = "3", [18] = "2", [19] = "1", [20] = "Begin!", ["GameTime"] = 235, ["Location"] = CFrame.new(36.548, 8447.98, -1174.169)
}

local function onPlayerAdded(player)
    print("Player: " .. player.Name)
end

while true do

    if tick() - tickRecord >= 15 then --waits 15 seconds
        if counter == 0 then -- only does this once
            m = math.random(1,1)
            game.ServerStorage.ObbyMap:clone().Parent = game.Workspace
            showingText.Visible = true --make it visible
        end
        if m == 1 then -- I just left this here but it'll probably be changed at some point
            if minigameText[counter] then -- Checks to see if the amount of seconds matches a key in the table
                showingText.Text = minigameText[counter] --This will change the text according to whatever is inside the minigameText table
                if counter == 20 then --Begin the game
                    for i, player in pairs (Players:GetChildren()) do
                        --Make sure the character exists and its HumanoidRootPart exists
                        if player.Character and player.Character:FindFirstChild("HumanoidRootPart") then
                            --add an offset of 6 for each character
                            player.Character.HumanoidRootPart.CFrame = minigameText.Location + Vector3.new(0, i * 6, 0)
                        end
                    end
                    wait(.5) -- otherwise it will instantly make the gui invisible, it'll desync the counter by half a second
                    showingText.Visible = false
                end
            elseif tick() - tickRecord >= minigameText.GameTime then -- Checks to see if the amount of time passed is equal to 200 seconds
                --Game ending, this section could use a cleanup but I got tired of coding lol
                counter = -1
                tickRecord = tick()
                showingText.Visible = true
                showingText.Text = "Winner!"
                for i, player in pairs (Players:GetChildren()) do --Teleport Players back
                    --Make sure the character exists and its HumanoidRootPart exists
                    if player.Character and player.Character:FindFirstChild("HumanoidRootPart") then
                        --add an offset of 6 for each character
                        player.Character.HumanoidRootPart.CFrame = CFrame.new(0,0,0) + Vector3.new(0, i * 6, 0)
                    end
                end
                wait(3)
                showingText.Visible = false
                game.Workspace.ObbyMap:Destroy() --Use destroy instead of remove
            end
        end
        counter = counter + 1
    end
    wait(1) --Delay by ~1 second
end

Players.PlayerAdded:Connect(onPlayerAdded) --This fires the onPlayerAdded function each time a player is added

I tested it and it works just fine. I think there may have been a problem with your pre-built gui/frame. Usually I don't go through this much trouble to help someone, but eh I was bored lol.