As @rpkamp said you need to transition from a left position as position itself is not an animatable property. However left:0 on a relative element has no bearing on a left position of a fixed element unless they just happens to live in the same place.
Usually you would avoid transitions by position values as they are jerky because they cause the page to be redrawn. Instead it is much better to use transforms when you animate as they have no impact on the flow and are much smoother. Use translate instead when you want to slide something into position.
Usually you position the element where you want it and then use a transform to move it off screen. Then you can just transform it back to where it should be.
It does depend on situation but if possible you should avoid transitioning position values.