The problem might be that you have applied a fade out effect, so in order for you to have that plain white background that you want, just select the 1st effect which is the default.
There is also another way of getting a white background which involves using one of the Shapes. You would go to Graphics > Shapes and select one.
Then drag it so it covers the whole canvas area of your banner. It will be black at first but you can change it to white by clicking on the Shape and choosing FFFFFF from the Color Option.
Then select your Shape again and keep clicking the "Send Backward" option from the Nav Bar until it goes to the very back of your Ad Design.
It's a little more work than using the Effects solution but may also come in useful for you.