The player mouse get locked after the BillBoardGui gets enabled:
Lua(u)
local UserInputService = game:GetService("UserInputService")
local Players = game:GetService("Players")
local player = Players.LocalPlayer
local Event = game.ReplicatedStorage:WaitForChild("EnterPc")
Event.OnClientEvent:Connect(function()
local gui = player:WaitForChild("PlayerGui"):WaitForChild("PcGui")
player.CameraMode = Enum.CameraMode.Classic
task.wait(0.5)
gui.Enabled = true
end)