Your scanner must be a TWAIN compliant scanner that can acquire a black and white image or grayscale image. A common problem is that you have installed an old 16 bit driver that can't communicate with 32 bit applications like SimpleOCR. Try to get an updated driver by downloading it from your scanner manufacturer's website.
If it still doesn't work, you can always scan from your scanning software, save the resulting image in a black and white TIFF file, and process the file with SimpleOCR.