[AccessD] Datasheet Subform column width a unhappily resetting

I am getting a message that "operation must use an updatable query" from
attempting to run this SQL. The reason I decided to try to join on a query
is that TblSites has tremendous duplication in it, and I would update based
on the TblSites table and it would be reported that many times more rows in
Tbl_Matched_Sites were about to be updated than in fact there even were in
the table. So I thought I might be able to improve performance by changing
the join to a group by query. Is there a way to get this to work?

UPDATE Tbl_Matched_Sites INNER JOIN 

(Select 
          FirstOf(Site_Station_Name) as FirstOf Site_Station_Name,
          First(Address_Line1) as FirstOfAddress_Line1,
          First(City) as FirstOfCity,
          First(State) as FirstOfState

>From 
          TblSites

Group By
          Site_Station_Name,
          Address_Line1,
          city,
          State  )

as   MainInfo  ON


Tbl_Matched_Sites.GIB_SITE_DB = MainInfo.FirstOfSITE_DB 

SET 

Tbl_Matched_Sites.GIB_Phys_Addr_1 = MainInfo.[ FirstOfAddress_Line1], 
Tbl_Matched_Sites.GIB_Phys_City = MainInfo.[ FirstOfCity], 
Tbl_Matched_Sites.GIB_Phys_State = MainInfo.[ FirstOfState]