Why my AI character can't move

My AI can’t move at all. I watch a video from Youtube teaching me how to get my AI charactaer move randomly.[link text][1]. I do exactly the same as the video, my bluprints are right and I have a navmesh setting properly. But when the game get started, the AI character was stucked and just stayed aroud doing nothing. I can’t see where it goes wrong and I have already spent a lot of time to check this out which is driving me crazy. I have attached some screenshots of my Behavior tree and Blueprints.
If anyone have any idea about this, I will really appreciate that if you just let me know.Hopefully someone can help me out. Save the kids,plz.