There is a NOx control requirement for CCS because low NOx is needed for it to work properly, and our models assume low NOx. The fact that older versions of the IECM allowed CCS without NOx control was an oversight on our part; the model was not designed to be used that way. (Some CCS options require SO2 control for similar reasons.)
If you need to simulate CCS without NOx control, you can select SCR and manually zero out its influence, i.e. set capital cost amortization to 100%, zero out O&M costs, removal efficiency, reagent use, etc. Note, however, that the results may not be accurate, as this is outside the scope of the model.