Can you try the following:
1. delete the line that reads:
const PlacementMode = AssetPlacerState.PlacementConfigState.PlacementMode
2. replace every instance of
PlacementMode
with
AssetPlacerState.PlacementConfigState.PlacementMode
I am wondering how that conflict comes to be. Can you tell me, which system are you on? Also, it might help if you could provide a copy of your .godot file, to see which settings you have. Does the plugin work in a new and clean project with default settings?