Are you trying to buy a revenue ticket or is this a Flying Blue redemption? Even though AF/KL are in the same alliance as MF, it is quite possible that the itinerary you are trying to get ticketed by AF/KL is something they just don't sell. There are a myriad of fare rules that apply in terms of combinability of fares, and it is possible that you are just falling out of those.

You are also trying to string together a very complex itinerary, YYZ-CDG-ICN-XMN. This is not a normal route so it is unlikely that a website will offer this to you. Do you have a specific reason to want to take this exact route? Or are you aiming to get to XML on a AF/KL issued ticket? If it is just that you want to have a AF/KL issued ticket, then YYZ to XMN is definitely possible on a combination of AF, KL and MF. I have never seen a fare that combines an AF flight from CDG to a MF flight, but there is a MF flight from AMS to XMN that regularly gets sold by KL as a KL coded flight. I have even flown on this service before. As an experiment, I tried looking up possible airfares and see that ITA Matrix can price a YYZ-CDG-AMS-XMN with a XMN-AMS-YYZ return.

That said, if this is a redemption ticket then this is very unlikely to work. Not every possible flight combination is available for redemption. You could try calling FB to see if they can get you a more complex redemption, but it might not be worth your points.