How do I fix the animation?

Try changing "is S down" to "on S pressed" (where you're calling the walk animation).

Currently you're starting your animation from the beginning every single tick. As long as you're pressing the S key, the "is S down" condition is true every single tick (sixty times a second or more). It looks like it's not playing at all, but it's just restarting so fast that you never get to see beyond frame 0.