Place inside StarterPlayerScripts

game.Players.LocalPlayer.CharacterAdded:Wait()
wait(1)

local Mouse = game.Players.LocalPlayer:GetMouse()
local Camera = game.Workspace.CurrentCamera
local defCFrame = Camera.CFrame --Default CFrame without mouse value

local function Update()
    local mx = Mouse.X / 1000 --Reduce value for faster movement
    local my = Mouse.Y / 1000 --Reduce value for faster movement
    Camera.CFrame = defCFrame + Vector3.new(mx, -my, 0)
    defCFrame = Camera.CFrame - Vector3.new(mx, -my, 0)
end
game:GetService("RunService").RenderStepped:Connect(Update) --Update Camera.CFrame every frame