I'm not sure if I understand your question correctly...

What I got is that you try to do a dummy booking for your flight to look at the seat map when you get to that part, no seat map is offered.

If that's the case, I would venture to say that it probably in an oversell situation (the Y9 B6 availability also points to this direction) with no more seats left to be assigned (at least to pax with your status level). UA is willing to oversell most flights--particularly with refundable Y & B fares, as these are often no shows, and if they do show up, there are always misconnects, op-ups, VBDs and IDBs (in that order) to accomodate everyone and get the flight out.