Why is this overdefined? Can't I translate eveything yellow horizontally to the right?
The coincident relation at the bottom of the line on the right will prevent you from moving it horizontally. It is overdefined because your
20.75 dimension is probably too short or too long to reach the corner that the right-hand yellow line is coincident too. Your line is also vertical, meaning that it cannot move only the upper point of the yellow line to satisfy the dimension.
Hope this helps!