To Create Rebar for a Structural Column, first of all you have to:
1. Create Rebar Configuration.
2. Link configuration to the column Type by using Column Link.
If you have already done that, select columns and hit Create Rebar.
It will create reinforcement for all selected columns based on linked configuration:
Another sequence for creating rebar is:
1. Select Create Rebar command.
2. Select Structural Columns. (If other elements are selected, the tool will automatically filter columns.)
3. Click Finish at the top left of working window.