First of all, do you only have an animation player in your scene ?
Animation player works with a SpriteSheet ideally, on a Sprite Node.
If you only have one animation, you could use an AnimatedSprite instead. It's simplier to use, but you have less control over it.
You can also use AnimationTree to append animation together, but that's only if you have multiple animations.
This one is for AnimationTree.