Your screenshot says “unknown skill”, without an actual skill id in it. It should read “unknown skill xyz” (where “xyz” is the invalid skill id).
The “place monster” action requires two skills. What did you wrote as second skill? An empty string? If so, try the “null” instead, it should be recognized as valid id. If that doesn’t solve it, please provide at lest the JSON for the item action.
Btw. if the “arcana” skill was unknown, the message would read “unknown skill arcana”.