I want to click the button on the Main UI to run Level Tools Utility Widget, I try to do it on blueprint, but it is not working.
Do I miss any stages?
You need an object reference from the Cast To LevelTools. You can in this case use a Get Actor From Class and plugin the result into the object. If required get a copy of the first index from the Get Actor From Class.
You will be using casting quite a bit when working in Unreal Engine, so I suggest reading or watching a few tutorials about the subject.
Edit: To further explain the reason why you are running into a Warning: If you think about it… there is no reference to the actual LevelTools which you have provided. Because you can have multiple LevelTools windows opened, the game engine is unable to know which one you want to modify.