I’m using Nav2 stack humble release which uses v.3.8 of BehaviorTree.cpp and I’m getting an error when using in a BT the following:
<SetBlackboard value="{goal}" output_key="orignalGoal" />
I get:

Behavior tree threw exception: missing port [value]. Exiting with failure

But the {goal} is initialized and used by Nav2 and in my BT without any problem for example
<ComputePathToPose goal="{goal}" path="{path}" planner_id="GridBased"/>
So I would expect that the value should not be missing in this example above.

In the Nav2 src I think this is the line which initializes the {goal}

...
bt_action_server_->loadBehaviorTree(bt_xml_filename))
...
// Update the goal pose on the blackboard
blackboard->set<geometry_msgs::msg::PoseStamped>(goal_blackboard_id_, goal->pose);

The bigger context is given in ros-navigation/navigation2#4016