i don't why the cube looks very dark if I see it from the top, but it looks pretty good from the botton. On the right you can see that my bounces are set well. In the second picture you can see my HDRI light settings. What should I do?
It looks like a shading problem, you've shaded smooth your object, which creates this unwanted shading on the big faces, just activate the Auto Smooth option and it will work fine.
Actually it happens because you're using a Solidify modifier, which virtually creates a 90° angle at the top, and therefore the shading between the side faces and these top faces. If you deactivate this modifier, the bad shading will disappear, if you keep it, keep the Auto Smooth.