When I want to export my artwork, this is positioned in the left margin. How to do to be centred? I'll add a screenshot to explain that better. Thank you!
That black area must be there for a reason.
You may not see it, but you probably have some leftover artwork/objects on the outside of the artboard and Illustrator is trying to produce a PNG that includes everything it can find in your file, including objects that you may not immediately see.
Especially if you got this file from somebody else, and/or it comes from another version of Illustrator, or it is made in Inkscape or some other software, you can get hidden objects around in the artwork.
I would go back to the artwork file, go to outline mode using CTRL+Y and see if there's any hidden or white object to the right of your artwork.
Sometimes you accidentally click with the Type tool and change your mind, cancel the typing, and still, you get an object created that trails and can generate such issues on export.
Things like that.