Ah, you didn't mention the bit about this being an extension in your original post.

This is because CO automatically extends returns by 6 months to Oct 16.  If your client owes nothing, there is no extension to request.

In the event your client has a balance due, 90% of the balance due would need to be paid in to avoid a late payment penalty but DR 0158-I cannot be efiled by any tax software, AFAIK.  (CO has a few quirks, as you may already know - to claim other state tax credit, you actually need to upload the other state's return on their website separately).

We normally would advise clients to just make an online payment on the CO DoR website if we expect that they would have a balance due.