What's the purpose of the Goldman equation?
I think that statement you posted from Kandel is a bit misleading, though not incorrect - I think he's just warning against a pitfall he's seen students make.
The Goldman Equation is an equation for steady-state voltage or in other words the reversal potential, not for Vm. It will only give you Vm in a (mostly) steady-state condition, like in the context of a resting membrane potential. However, that doesn't mean it isn't useful to calculate the Goldman equation voltage when Vm is changing or that it doesn't apply, but rather that it is not sufficient to use. It will still accurately tell you what the reversal potential is at any moment in time, but it can't tell you what Vm is at that moment.
How the Goldman equation can be helpful in understanding dynamic voltage changes:
Importantly, the rate of change of the voltage of the cell is going to be proportional to the difference between the reversal potential (from the Goldman equation) and the current voltage Vm, and after a long time the voltage will eventually asymptote to the Goldman voltage.
Therefore, if you know the current Vm, and you calculate the reversal potential with the Goldman equation, you can make some inference about what will happen to the voltage at the next moment in time: it will change in the direction of the reversal potential by a magnitude proportional to
Vm - Ereversal
Why the Goldman equation is not sufficient while Vm is changing:
I suspect what Kandel and others are warning about is that this isn't quite enough information to know what happens in the next moment of time:
It takes time to reach a steady voltage, and you need to know the membrane capacitance, which isn't represented in the Goldman equation, to know what the actual change in voltage is after $\delta$t, or how long it will take to reach a particular voltage.
Also, because of voltage-dependent conductances in a neuron, the permeabilities to different ions are also changing as a function of Vm, so the reversal potential is also changing constantly.
Furthermore, simulations of Vm sometimes need to consider the changing concentrations of individual ions, because the reversal potential will change as ion concentrations change. Therefore, it typically makes more sense to think about the driving force of individual ions given by the Nernst equation rather than the Goldman equation which gives you the net reversal potential across all ions.
In summary, authors that warn against using the Goldman equation when Vm is changing are reminding you that voltage changes take time and depend on membrane capacitance as well as changes in conductance from voltage-gated channels. However, the Goldman equation is still a correct way to calculate the instantaneous reversal potential at any moment in time (but not Vm), even if Vm is dynamic.
You could edit Kandel to say:
The Goldman equation only outputs the present Vm when Vm is not changing; also, when Vm is changing, the output of the Goldman equation is also changing due to changing conductances and ion concentrations.
..but you can see this is a bit verbose.