Simple :) create an "else:" statement at the bottom of your if statements. Your code should look like this:
if Input.is_action_pressed("move_up"):
vel.y -= 1
facingDir = Vector2(0, -1)
$LilBoiSprite.play("WalkBack")
if Input.is_action_pressed("move_down"):
vel.y += 1
facingDir = Vector2(0, 1)
$LilBoiSprite.play("WalkFor")
if Input.is_action_pressed("move_left"):
vel.x -= 1
facingDir = Vector2(-1, 0)
$LilBoiSprite.play("WalkLeft")
if Input.is_action_pressed("move_right"):
vel.x += 1
facingDir = Vector2(1, 0)
$LilBoiSprite.play("WalkRight")
else:
$animationplayer.stop()
so what your code is saying is " if whatever input I pressed is true, play animation.
otherwise if I'm not pressing an input, stop my animation."
Also I recommend changing your other "if" statements under your first "if" statement to "elif instead. This will allow your code to override your previous button presses and keep your animations from not playing properly. Hope That solved your problem! :)