The code isn't working it won't change the text label?

Your problem is that the game doesn't know whats playerWhoClicked so it doesn't know what to do and you get an error.

I'll do the work for you:

Script in ServerScriptService:

local part = game.Workspace.Part
local whoClicked = game:GetService("ReplicatedStorage").whoClicked

part.ClickDetector.MouseClick:Connect(function(playerWhoClicked)
    whoClicked:FireAllClients(playerWhoClicked)
end)

Script in ScreenGui:

local whoClicked = game:GetService("ReplicatedStorage").whoClicked

whoClicked.OnClientEvent:Connect(function(playerWhoClicked)
    game:GetService("Players").LocalPlayer.PlayerGui.ScreenGui.TextLabel.Text = playerWhoClicked.Name
end)

If this works please accept my answer, It helps me and you