Why is this overdefined? Can't I move right vertical line to the rright?

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!