Try statically typing your vars, like this:

var party_scene := preload("res://Resources/Party Members/Party Member.tscn")

func createPartyMemberObject(root, job):
  var party: PartyMember = party_scene.instantiate()
  ...

Source.

Try statically typing your vars (and func return value), like this:

var party_scene := preload("res://Resources/Party Members/Party Member.tscn")

func createPartyMemberObject(root, job) -> PartyMember:
  var party: PartyMember = party_scene.instantiate()
  ...

Source.