Book Creator is designed to work on laptops, PCs and most tablets. If you tablet is too small (or you try to go to app.bookcreator.com on a mobile phone) you will see the message 'Device not supported'. Book Creator will only work on devices with a screen resolution greater than 600px wide and 570px high.
Even if it looks like your screen resolution is correct, perhaps the computer has been configured to a different screen resolution in your display settings? You can check using this website: whatismyscreenresolution.com.
You'll need to access the display settings for your device to change the resolution, or check that you haven't set your operating system to increase scale.