If you don't have a B/Reserve Team or if you don't have an assistant manager you won't be asked. If you have a reserve side or youth team you can still schedule a game between the sides the same way you arrange every other kind of friendly match from your fixture screen.
Playing as Arsenal. It's not a huge deal as stated above I can easily arrange it myself, just a personal annoyance that it just suddenly seems to have stopped asking me when I start a game and I wondered it if was just me or not?