local TweenService = game:GetService("TweenService")
local RA = script.Parent.Parent
local rah = script.Parent
rah.Parent = RA
rah.CFrame = RA.CFrame
rah.Weld.Part0 = RA
rah.Orientation.Y = rah.Orientation.Y + Vector3.new(0, 130.68, 0) -- This is the line that errors
TweenService:Create(rah, TweenInfo.new(1.3), {Size = rah.Size * 2, Transparency = 1}):Play()
game.Debris:AddItem(rah, 1.3)
I just want the part to rotate its orientation a bit but it errors.
It's a mesh part inside of a character's right arm.
The code above is inside a server script
There is a weld instance inside the mesh part which has the weld Part1 set to the mesh part and of course, the script sets the weld Part0 in the script above.
The tween just makes it grow a bit then disappear.

2 comments
Orientation is a vector, you get the Y axis of that vector (so basically you get a number), you try adding the number to a new vector and furthermore errors
rah.Orientation = rah.Orientation + Vector3.new(0, 130.68, 0)seems good for me as both are vectors now