This is a bit above that code:
So
"SpawnPath/SpawnLocation" is indeed the correct path, you probably didn’t set up your node tree exactly as it is in the documentation.
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Your supposed to replace all the preexisting text under # Question with your question:
In this tutorial page:
Spawning monsters — Godot Engine (stable) documentation in English
The following line occurs. It fails:
var mob_spawn_location = get_node("SpawnPath/SpawnLocation")
But this succeeds:
var mob_spawn_location = get_node("SpawnLocation/SpawnPath")