I want to create a destruction physic for my mesh but I cant cast to Destructible Actor, I am using 4.18.
You cannot create a Destructible Mesh just by casting a regular mesh. What you need to do is right click in the browser on a mesh you want to make destructible and choose Create Destructible Mesh. You will get a new mesh which is destructible while the original one is non destructible.
Here a useful reference: How to create a Destructible Mesh in UE4 | OnlineDesignTeacher