Message command not ending/showing?

This is based on the assumption that there are no other parts to this script. So, technically, yes, your script would play, but only after the first person to ever start that particular server joins the game. It will never play again after that. Also, your script doesn't correctly start when a player joins, it starts when the server starts. This means that the player most likely won't see the entire message since your script will load before the player does. For a very basic setup, you're going to need tick() to account for the time passed since the server started (since someone did join to start it) and a while loop to keep running your game. You also need to take into account that the first person who joined may have left within 21 seconds of your first message, so go ahead and make sure there's someone even in the game to start it.

local msg = Instance.new("Message", workspace)

local minimumPlayers = 1
local startCount = 5
local stopCount = 0
local incBy = -1
-- You can also do this => local startCount, stopCount, incBy = 5, 0, -1
local pause = 1 -- your wait between messages
local serverStartTime = tick()
local delayedStart = 21



local function start()
    msg.Parent = workspace
    for num = startCount, stopCount, incBy do
        -- returns true since it's the first run
        local firstMessage = (num == startCount) 
        -- if firstMessage == true then wait is 5s else wait is 1s
        local waitTime = firstMessage and 5 or pause
        -- %s inserts your variable with string.format(), in this case "num"
        msg.Text = string.format( "The game will be starting in %s", num)
        wait(waitTime)
    end
    -- parent the message to nil, so you can re-parent it back when you need it
    msg.Parent = nil
    -- move on to next function, else it goes back to the while loop
end


while true do 
    -- Comes back to this check after all functions have completed / game has ended
    local timeUntilStartable = ( tick() - serverStartTime )
    -- prints the delay countdown in the output
    print(timeUntilStartable < delayedStart and math.floor(timeUntilStartable) or "We can start when there are enough players")
    if game.Players.NumPlayers >= minimumPlayers and timeUntilStartable >= delayedStart then 
        start()
    end
    wait(1)
end