Make sure you set the coordinates X and Y of the Bounding box properly.
The bounding box is supposed to be the editable area on the canvas. So the coordinates allow you to define the position of that area on the canvas. If you set a couple of coordinates that put the box outside the canvas, nothing will be visible on the canvas.
Please also note that these parameters are mandatory if you need only a specific part of the product in the canvas/custom page to be editable. If you don’t define them, the box will be disabled.
If you don’t have a specific values to set or not sure of what values to set, it is advisable you do not define them. As said earlier, setting the wrong coordinates or values might put the box outside the canvas thereby making anything being added in the canvas not visible.