Unfortunately, the front wheel of the Monaco can sometimes be placed on the frame the wrong way round. If the wheel is upside down, the front wheel cannot be locked.
If the front wheel is upside down on the Monaco, it can also be very difficult to remove it from the frame.
In the video below, a metal rod is used to push in the lock on two sides. You could also try doing this with the handle of cutlery or a chisel. (see photo)