I want to animate the cube over the ui panel.
The Canvas is set to Render Mode : Screen Space - Overlay The Panel have my own source image The Camera Clear Flags set to Skbox and Culling Mask to UI only The Cube layer is set to UI
But the Cube is transparent and I want it to be full like a regular Cube.

If the gui is in overlay mode it’ll render on top of every 3D object. Set it to either to worldspace or screenSpace: camera (the second one will follow the camera, so it’ll give a static impression while not rendering on the top of everythiing). Regular UI goes in overlay because it renders on top of everything by default.
To decide wich of your UI elements is render in front and wich on the back you put it either a Canvas or a Canvas Scaler and set it’s “order in layer” value. The higher the value, the more in the front it’ll render.
Ultimately, the cube layer shouldn’t be set to the UI layer as it’s not really a UI element. If you want a 3D object to act as part of the UI, like, say, your character’s model, this is usually achieved by using a second camera
I think it’s because of image script in panel. You can add mask component to panel from inspector and uncheck it or just make it transparent by image’s color.