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 Configurations.

3. Assign configurations to wall layers by using Wall Link.

4. Select Wall(s) with that Type (used in Wall Link) and use Create Rebar command. It will read from Wall Link what configurations of reinforcement should be used and will create rebar.