RigidBody2D mode change

[stiff elm]

Are you doing something different from that tutorial, with regards to when pickup() is called?

You won't be able to change the body's mode during any physics callback function

[slim furnace]

Godot 3.5 prevented people from scaling rigid bodies

And turning them off during physics process fps

Using set_deffered() prevent it from taking action in current frame

It is like queue_free()

[stiff elm]

I'm sure you must be doing something slightly different from the tutorial. How are you calling pickup()?

[stiff elm]

You can't change the physics state at all during a physics signal callback. If you need to, you will need to defer the call