How do I fix this number randomizing script?

game:GetService('Players').PlayerAdded:connect(function(player)
	player.CharacterAdded:connect(function(character)
		character:WaitForChild("Humanoid").Died:connect(function()
			local value = script.Parent.Value
			value.Value = math.random(1,10)	
			if value == 6 then
				script.Parent.Parent.ScreenGui.NewItem.GreenSaber.Visible = true
			end
		end)
	end)
end)

I confused myself and all I want to do is... 1.) When a player dies, randomize a number 1-10 2.) If it is equal to 6, make a GUI visible.

You've got a bit mixed up with your "value" variables.

On line 4, you declare "value" as script.Parent.Value, I think that this is an IntValue? Meaning that the stored variable will be an integer.

You then try to change the value of the integer variable, which of course, there is none.

You will want:

game:GetService('Players').PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").Died:connect(function()
            local value = script.Parent --Set the variable to the IntValue instance
            value.Value = math.random(1,10)  --Set the IntValue's value to the rgn
            if value.Value == 6 then --Check the value
                script.Parent.Parent.ScreenGui.NewItem.GreenSaber.Visible = true
            end
        end)
    end)
end)

Unless you need the IntValue's value, I'd recommend using the following script and assigning a local variable instead:

game:GetService('Players').PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").Died:connect(function()
            local value = math.random(1,10)  --Set the variable to the rgn
            if value == 6 then --Check the variable
                script.Parent.Parent.ScreenGui.NewItem.GreenSaber.Visible = true
            end
        end)
    end)
end)