Please help. I don't why it doesn't work

You have lots of problems.

• First, you are looking at the Layer Panel and that is what you use to do Motion Tracking so the other layers in the comp won't show up 
• Second, the Track point is not positioned over any detail so it won't track anything
• Third, if this comp is from Character Animator you might have rendered an image sequence or just imported part of your character because I can only see the arm, and that is not a normal workflow
• You don't seem to be using the Motion Tracking workspace and that would simplify the UI for you and give you access to all of the tools you need
• Fourth, you are either following a poorly prepared tutorial by an enthusiast or you are trying to work in After Effects without spending any time in the Learn Workspace or with the User Guide so you have no idea how the basic UI is laid out or how it works

After Effects is an extremely complex application. It's nearly impossible to diagnose problems from cropped screenshots that don't show the modified properties of the layers you are working with. Select the layer, press uu, and give us an uncropped screenshot.

If I was going to motion track a character's hand Character Animator this is how I would set up the tracker. The feature region, the Inside Box, needs to include some significant detail so I am including most of the hand. The Search Area, the outer box, needs to be big enough to keep the hand inside the search area when it moves between frames. The bigger the Feature Region and the bigger the Search Area, the slower the tracker will run. The attach point, the little +, needs to be centered over a detail that doesn't change so you can edit the motion track easily when the track gets off. Even with a big Feature Region and Search Area, the track failed four times with a simple hand wave. Maybe this will help, but you get farther quicker if you spend a few hours learning the basics.

I would probably use Mocha AE to do this kind of tracking instead of AE's motion tracker because it tends to be much more accurate, but I probably wouldn't use this workflow if I wanted to attach something to the hand of my puppet in After Effects.