[LunaPlaygroundAsset] Attribute added onto "FIELD NAME" field of "CLASS NAME" class must have a default value to appear in Playground.
The sole purpose of a field with
[LunaPlaygroundAsset] is to hint Playground that a specific asset should be modifiable via Playground UI. The field with this attribute does not have to be considered by game code in any way (although developers are free to do so if needed). Obviously, for the Playground to know which asset to modify the field has to point to some existing asset. Thus, a field marked with such an attribute and lacking an association to an asset will not function properly.
Steps to fix
Please assign an asset to the field via Unity's inspector window.