Making your Player node an Autoload will effectively load it twice into the scene:
Inside your main scene, as it would normally
Under the root node, as an empty Node with just the player.gd script, because you set the player.gd script as an Autoload (see the error message refering to the Player node under root (relative to “/root/Player”))
Because the autoload node is empty, it doesn’t have any of the children you set up on your player node initially, like $AnimatedSprite2D, so now any pointers related to these will just return null - and this is the error you’re having.
Just remove the player from your Autoload setup. Basically Player should never be an Autoload, this seems like a bad practice. Setting only a .gd script file as an autoload is also a bad practice, you should make a .tscn scene file an autoload instead, if you need it (but not the Player).
What’s the reason you set the Player up as an Autoload? I’m sure we can find a better solution for that.