Select a slab and at least 1 beam supporting this slab to open the extension.
Only wide-flange shapes (W-Shapes) can be designed.
They are available in the AISC 14.0 (American hot rolled shapes) database you need to select in Robot before you open the extension.
Non W-Shapes (such as Channels) can be selected if they bear on beams to be designed. They are not designed but they are used to determine load on the designed beam.
Anything in the selection other than beams is filtered out automatically.