After i added the solidify modifier the sphere turns black

In the Render properties under Light Paths, try turning up the Max Bounces for Glossy and Transmission to at least 8. That should do it! 

You're not doing anything wrong! We'll cover this later in the course but at a certain angle light can't refract, which is why it looks black around those edges. We will later fix this by making those edges reflective rather than refractive.