Hi. I suspect because Unity is super weird with their navmesh stuff it seems like nobody ever made a proper way to notify you in one action in PM to when your agent finished their path.
That's probably because the answer is weird, you do it like this in c# checking two bools for false:
using UnityEngine;
namespace HutongGames.PlayMaker.Actions
{
    [ActionCategory(ActionCategory.NavMeshAgent)]
    [Tooltip("Sends an event when an agent has completed their path (once AI bools hasPath and pathPending are false.) \n" +
        "NOTE: The Game Object must have a NavMeshAgentcomponent attached.")]
    public class AgentArrivedAtDestination : FsmStateAction
    {
        [RequiredField]
        [Tooltip("The Game Object with the Nav Mesh Agent.")]
        public UnityEngine.AI.NavMeshAgent agent;
        private bool checkHasPath;
        private bool checkPathPending;
        [Tooltip("Next event when agent is done with their path.")]
        public FsmEvent agentArrivedEvent;
        [Tooltip("Repeat every frame. For normal purposes this should be ON.")]
        public bool everyFrame;
        public override void OnEnter()
        {
            checkPathPending = agent.pathPending;
            checkHasPath = agent.hasPath;
            if (everyFrame)
            {
                return;
            }
            else if (!checkHasPath && !checkPathPending)
            {
                Fsm.Event(agentArrivedEvent);
            }
            else
            {
                Finish();
            }
        }
        public override void OnUpdate()
        {
            checkPathPending = agent.pathPending;
            checkHasPath = agent.hasPath;
            
                if (!checkHasPath && !checkPathPending)
                {
                    Fsm.Event(agentArrivedEvent);
                    Finish();
                }
        }
        public override void Reset()
        {
            agent = null;
            agentArrivedEvent = null;
            everyFrame = false;
        }
    }
}
Works fine for me using it, but let me know if there's any issue I haven't seen.
edit: cleaned up something that was redundant in code