GoalSeek will throw an "Invalid Reference" error if the GoalSeek cell contains a value rather than a formula or if the ChangingCell contains a formula instead of a value or nothing.
The GoalSeek cell must contain a formula that refers directly or indirectly to the ChangingCell; if the formula doesn't refer to the ChangingCell in some way, GoalSeek either may not converge to an answer or may produce a nonsensical answer.
I tested your code with a different GoalSeek formula than yours (I wasn't quite clear whether some of the terms referred to cells or values).
For the test, I set:
the GoalSeek cell H18 = (G18^3)+(3*G18^2)+6
the Goal cell H32 = 11
the ChangingCell G18 = 0
The code was:
Sub GSeek()
With Worksheets("Sheet1")
.Range("H18").GoalSeek _
Goal:=.Range("H32").Value, _
ChangingCell:=.Range("G18")
End With
End Sub
And the code produced the (correct) answer of 1.1038, the value of G18 at which the formula in H18 produces the value of 11, the goal I was seeking.