Ericpro06 - Use this, your events above have conflicts:
*KeyBoard - On Left Arrow Pressed -
-----> Sprite --> "Set animation to "Walk" (play from beginning)
-----> Sprite --> "Set Mirrored"
*KeyBoard - On RIght Arrow Pressed
-----> Sprite --> "Set animation to "Walk" (play from beginning)
-----> Sprite --> "Set Not Mirrored"
*KeyBoard - Left arrow is down(Inverted) & Right arrow is down(Inverted)
-----> Sprite --> "Set Animation to "Idle" (play from beginning)
Tell me if there is something wrong because I haven't tested it but it should work.