attempt to index nil with 'FindFirstChild'?

The reason why this is not working is because you were accessing leaderstats from game.Players, rather than the actual player itself. When it says "attempt to index nil", it's basically saying it's trying to access something that doesn't exist, which in this case is leaderstats. Your leaderstats existed in the player, not the game.Players.

The fix for this code would be relatively simple. Just change where you access leaderstats.

ChangeOEvent.OnServerEvent:Connect(function(Player)
local Cpoints = game.Workspace.CheckPoints:GetChildren()
local obbyv =  game.StarterGui.InsertedObjects.ObbyButton.Value
local tycoonv =  game.StarterGui.InsertedObjects.TycoonButton.Value
local Teams = game:GetService("Teams")
local Cpoints = game.Workspace.CheckPoints  

local leaderstats = Player:FindFirstChild("leaderstats")
local S = leaderstats:FindFirstChild("Stage")
if Player.Team == Teams.Red then
    obbyv.Value = true
    tycoonv.Value = false
    if obbyv.Value == true then



        local hum
        Player.PlayerGui.InsertedObjects.ObbyButton.BackgroundColor3 = Color3.new(0.545098, 0.545098, 0)
        Player.PlayerGui.InsertedObjects.ObbyButton.Shadow.BackgroundColor3 = Color3.new(0.282353, 0.282353, 0)

        Player.PlayerGui.InsertedObjects.TycoonButton.BackgroundColor3 = Color3.new(255, 0, 0)
        Player.PlayerGui.InsertedObjects.TycoonButton.Shadow.BackgroundColor3 = Color3.new(0.290196, 0, 0)
        local char = Player.Character
        if char then
            hum = char:FindFirstChild("Humanoid")
            Player.CharacterAdded:Connect(function(hum)
                hum:MoveTo(Cpoints[S.Value].Position)
            end)
        end
    end
end

I removed the players line, because it's unneccessary, since you're not using the player list, you're getting the player straight up from who fired the remote event.