Why can't I add to this number value?

Where do you create Players.Slaughters.Value? on the server or on the client? wherever it is created that is the only place it can be changed. If it is created on the server, adding to it or printing it locally won’t show any change. Try adding/printing the Players.Slaughters.Value after you send the Value to the server in:

game.ReplicatedStorage.SlaughterUI_Events.Event2:FireServer(Value)

I’ve had this issue before. You might’ve accidently done something like this :point_down::point_down::point_down:

local Value = PlayerGui.MyValue

This is how it should look like :point_down::point_down::point_down:

local Value = PlayerGui.MyValue.Value