Got it sorted out. My codeshare was a United booking on AC metal. Called AC to check on the seat selection and they said its not confirmed and blocked. They told me to call UA to get seat selection. The UA agent was able to give me the seats I wanted. Called AC back to check and the seat assignments are confirmed.

You learn something new everyday. I thought seat selection requests on codeshare flights are supposed to made thru the airline you're actually flying on.