Hi and, again, welcome to FT.

Sadly, you don't have much choice here, apart from awaiting check-in to open. What you could do as a last resort option (if you haven't done so already, needless to say) is to call AZ and ask them to give you the AF booking code (PNR), then call the latter back and ask whether they would be kind enough to let you choose and pay for a seat through the phone. FYI and future reference - such mixed tickets involving AZ, AF and KL are always tricky when it comes to seat selection, check-in and the likes (there are several reports here on FT) and, not that this would make you feel slightly less annoyed, but you would have stumbled on the very same system's gremlin had you booked through AF and wanted to select seats on the AZ flight.

G