Has already said in Issues with the Inspector in progression
The inspector is broken when it’s come to nested array, and that make the Progression scriptable object impossible to use after the video " Editing Scriptable Object Files".
The jank solution to put a false element at the top is not enough at this point.
Luckily, another workaround is to use the [NonReorderable] attribute for every array inside the script.
Solution found here (the bug is well know) : https://forum.unity.com/threads/error-displaying-nested-arrays-in-the-inspector.1104037/