I don't know why that most people can turn on their onion skin by click the lightbulb in time line. But on my laptop it didn't show a lightbulb. How can I turn the onion skin on ?

In the Timeline docker, a layer will not have a lightbulb icon unless it is an animated layer. All layers start out as non-animated layers. So, first you need to make the layer animated.
In the Timeline docker, right-click frame-0 and then click the Create Blank Frame option. This will cause the layer to be an animated layer with frame-0 having no content. The layer will now have a lightbulb icon, because it is now an animated layer. Then, you can paint on frame-0. Then you can create more blank frames and paint on them and use the onion skins.

If instead of Create Blank Frame, you choose Create Duplicate Frame for frame-0, this will take the existing content of the non-animated layer and make it become the content in frame-0 of an animated layer.

The usual use of Create Duplicate Frame is to make a copy/duplicate of the previous frame and place it in the current frame.

If you want to make an animated layer into an ordinary non-animated layer you can do Flatten Layer on the layer in the Layers docker. This will make the layer non-animated with the content of whatever frame number was selected at the time.