There are a number of potential answers.
If you are not UA elites, the most likely answer is the regular part of economy is full -- however it is very likely the Economy Plus section is not full. In that case UA continues to sell seats and will place you in Economy Plus section (at no extra charge) if need be. If this is the situation you will not get you seat assignments until you get to the airport. You should be able to see the open E+ seats, if any, on united.com seatmap -- it will want to charge you to make a seat assignment now -- but if you wait till the airport there will be no extra cost.

It is possible all of economy is sold / over booked. This is far less likely to the first case (which is common in the summer when non-elite travel is up and elite travel is down). If it is this case, the first step will be to ask (and provide some compensation) for volunteers to fly later -- this usually resolves the situation. If that does not work, then there is a slim possibility you will be involuntary denied boarding (VDB) -- as an award ticket there will be no cash compensation that a paid passenger would get (so better to take the optional change) but you will be placed on the next available flight.