Well, to start with vel is a Vector2 and then you are setting it to equal speed, which is a number, so that doesn't make sense (and even if the types were the same, that wouldn't make sense).
So you probably want to remove the line "vel = speed". Probably the best thing would be to change "is_action_just_pressed" to "is_action_pressed" (so it happens every frame rather than just once). And at the start of "_process" set "vel = Vector2.ZERO".
See if that helps.
