What was the error?

The system says that it cannot be transmitted because the non-resident state return is a required PDF attachment.  However, the PDF has been attached, but the resident state return still wouldn't transmit.

I found a work around (marking that the required PDF would be sent separately), but not sure why Intuit wouldn't recognize the attachment and enable the filing.

When you attached the PDF did you label in the attaching that it was the other state return? The drop down selection defaults to "other" so the software does not know that you attached the required state return.