Assume LGW flights. Euroflyer seat maps havent been showing in the new MMB for a while now.
Switch to old MMB layout or use the app.
Yes, had this yesterday after booking a LGW flight for the first time since Euroflyer went live - switching to old style MMB and selecting the Seating option down towards the bottom of the page let me select seats (and they had already been allocated for me as a Gold, by the by)
SIGH