? is not a valid member of Folder

Hey there, I want to edit the value of an IntValue, that got created via other script. But everytime I try to open this script, "FullValue1 is not a valid member of Folder" shows up.

This is my code:

script.Parent.MouseButton1Click:Connect(function()
    local Players = game:GetService("Players")
local player = Players.LocalPlayer  
local path = game.Workspace.GameData.Player 
local GetName = player.Name
local FullValue1 = GetName .. "IsInGame"
print(FullValue1)
local fvalue = workspace.RealPath.Value 

        if script.Parent.Text == "Enter Game" then
            local sss = game.Workspace.GameData.Player.FullValue1.Value
            print(sss)
            game.Workspace.GameData.Player.FullValue1.Value = "out"
            game.StarterGui.ingui.Frame.Visible = false
            script.Parent.Text = "Exit Game"
        else
            fvalue = "in"
            game.StarterGui.ingui.Frame.Visible = true
            script.Parent.Text = "Exit Game"
        end
    end)