In destop mode appear the bottom to add arcticle.

In mobile mode desappear the bottom, where should go to add article?

Scroll down.

3 hours ago, Nathan Explosion said:

Scroll down.

nothing...

It’s true though. The “Add” buttons moves below the content on smaller screens. In addition, the button gets hidden, when the user doesn’t have permission to add an article (e.g. as guest). 

4 hours ago, opentype said:

It’s true though. The “Add” buttons moves below the content on smaller screens. In addition, the button gets hidden, when the user doesn’t have permission to add an article (e.g. as guest). 

but I use administrator account, and can not see the "add" button, only appear when I change to destop mode.

Well, what we describe is the default behaviour in the stock theme. Here is an example from the bug tracker:

See the green button that has moved below the content? That’s how it works by default. If it doesn’t on your site, then it might be a problem of your theme or something else. But we can’t tell that from just these screenshots.