How do I make a map changing script?

This should work. You had some pieces of code out of order, some parts that were extra, and some that needed just a slight adjustment. You did a great job of writing the first draft, the next thing you need to know is how to debug your own code and to double check thing against the wiki. Hope I helped, accept the answer and/or up vote if I did. Thanks!

local maps = {"Classic", "Large", "Medium", "Small"} --Name of the model, also known as the map
local time = math.random(300,600) --local variable lookup is faster, therefore reducing lag.
local script.Parent.TimeValue.Value = time --players is defined later

function ChooseMap()
    map = game.Lighting.maps[math.random(#maps)]:Clone() --Clones a Random Map
    map.Parent = game.Workspace --Puts the map in Workspace
end

while wait(0) do 
    local players = game.Players:GetChildren()
    if #players > 3 then --If there are more than 3 (4 or more) players, then…
        ChooseMap() --Chooses the map
        local sniper = math.random(#players) --selects a random player to assign to var 'sniper'
        players[sniper]:MoveTo(map.SniperSpawn.Position) --Choose sniper and send to sniper spawn.
        for i,v, in pairs(players) do --for each player do
            if v ~= sniper then --if player not sniper then
                v:MoveTo(map.PlayerSpawn.Position) --Players to player spawn
            end
        end
        repeat --countdown
            waitTime = wait(1) --record actual time waited when asked to wait for one second
            script.Parent.TimeLeft.Value = time-waitTime --subtracts the actual time waited.
            print(math.floor(time+.5) --this is what you should use to display time, if you do.
        until script.Parent.TimeLeft.Value <= 0
        wait(15)
        map:Destroy() --destroy the cloned map
        time = math.random(300,600) --resets the random time
    end
end