The list_work_item_relations, create_work_item_relation, and remove_work_item_relation tools all call a relation endpoint /work-items/{work_item_id}/relations that doesn't actually exist in Plane right now
There is an issue, makeplane/plane#6236 that is requesting to add an official relation management API.
It seems like these tools should be removed, or the above issue should be prioritized and working endpoints should be added.