Generally in order to ticket an itinerary, there needs to be at least one flight marketed (e.g. with the airline's flight number) by the "plating carrier" whose ticket stock the flights are issued on.

I can't buy an itinerary consisting of just BA238 on aa.com but I can buy AA6981 which is its codeshare. I can also buy an itinerary where I fly AA on BOS-JFK and connect to BA JFK-LHR, because there's at least one AA-marketed flight on the ticket.

The marketing carrier also can affect how the operating carrier gets paid -- codeshares can have different inventory which allows airlines that are partners but not super close to hold back inventory for themselves.