These cpp errors often disappear on their own; something will be missing or misordered the first time the project is run with a feature, but that first run “clears all the cobwebs out,” so to speak. This error, however, is a recognized issue, given that you’ve said you’re following the Godot 3D course.
I suspect you’re following Barbarian Blaster and that this error has to do with a known problem in the Path3D node. Ignore this for now, as Bram addresses it directly in a later lecture (I believe it’s something like 2 or 3 lectures from the end of the section). Your project will be fine in the meantime because the statement itself is largely just a warning and it overrides its own result: Returning: false [instead]. It won’t break anything =)