How to make a random player picker and teleport them to a block?

As I've mentioned in my comment in the other answer, the other answer is trying to index the child of a print() call, which obviously does not exist. Also, you're trying to set the CFrame property of a Player Instance.

Instead, it should be like this:

task.wait(5) -- `task.wait` is the "updated" version of `wait` and is more precise
local num = math.random(1, #game.Players:GetChildren())
local player = game.Players:GetChildren()[num]

print(player.Name)

local hrp = player.Character.HumanoidRootPart
hrp.CFrame = CFrame.new(Vector3.new(11.667, 23.652, 95.261)) -- Teleport to where you wanted
task.wait(45) -- Wait 45 Seconds
hrp.CFrame = CFrame.new(Vector3.new(0,0,0)) -- Teleport back to spawn

You gotta set the player to the random player

wait(5)
local num = math.random(1, #game.Players:GetChildren())
print(game.Players:GetChildren()[num]).Character.HumanoidRootPart
local player = game.Players:GetChildren()[num]
player.CFrame = CFrame.new(Vector3.new(11.667, 23.652, 95.261)) -- position
player = nil
-- teleporting to spawn
wait(45)
player.CFrame = CFrame.new(Vector3.new(0,0,0)) -- or whatever the position of the spawn is. You can also get the position of the spawn part to do this.

It's because you didn't set the player variable, just as what @JustinWe12 said.

Let's say you have Part named StagePart in Workspace and a SpawnLocation where all player spawns.

To teleport the Player to StagePart, you don't need to set Player.CFrame because the Player Instance doesn't have a CFrame. Instead, get the Player's character model using Player.Character and use its CFrame using Character:GetPivot().

To teleport the Player back to spawn, we simply do Player:LoadCharacter() because it respawns the player. When a Player respawns it will automatically teleport them to a SpawnLocation.

Also we should make a BindableEvent that will detect if Player left or died.

The script should look like this:

local Players = game:GetService("Player")
local StagePart = workspace.StagePart
local Spawn = workspace.SpawnLocation

local bindableEvent = Instance.new("BindableEvent")

function tpRandomToStage()
    task.wait(5) -- task.wait() is slightly faster than wait()

    local allPlayers = Players:GetPlayers() -- basically :GetChildren() but only choose Players
    local randomNumber = Random.new(os.time()):NextInteger(1, #allPlayers)
    local randomPlayer = allPlayers[randomNumber]
    local randomCharacter = randomPlayer.Character or randomPlayer.CharacterAdded:Wait()
    local randomHumanoid = randomCharacter:FindFirstChildWhichIsA("Humanoid")

    randomHumanoid.Died:Connect(function() -- when the player died
        bindableEvent:Fire()
    end)

    randomPlayer.CharacterAdded:Connect(function() -- when the player respawns
        bindableEvent:Fire()
    end)

    Players.PlayerRemoving:Connect(function(player) -- when a player leaves
        if player == randomPlayer then -- if the player leaving is the same as the player on the stage
            bindableEvent:Fire()
        end
    end)

    bindableEvent.Event:Connect(function()
        return -- stops/breaks the whole function
    end)

    -- PivotTo and GetPivot are both CFrame
    randomCharacter:PivotTo(StagePart:GetPivot()) -- teleports the player
    task.wait(45)
    randomPlayer:LoadCharacter() -- respawns the player and teleports back to spawn
end

tpRandomToStage()