Scribble option disappeared? - here's a fix.
In my case, the scribble option disappeared when I added Gboard to my iPad and deleted the default apple keyboard. This could be your case too. Just don't delete the keyboard that comes with iPadOS & Scribble should work fine. In case you have deleted it, you can easily restore it back by: Settings > General > keyboard> keyboards> add new keyboard
iPad, iPadOS 16