1.  Make sure you have Rebar Shapes in your model. They should be visible in your Project Browser:

If you don't have them, load them from Library

2. Prepare Rebar Configuration for selected beam section.

3. Assign configurations to Structural Framing Types by using Beam Link.

4. Select Structural Framing(s) with the Type used in Beam Link, and use Create Rebar command. It will read from the Beam Link which reinforcement configuration should be used and will then create rebar.