Following are some ways to fix scanning issue in Canon Pixma MG 7720:
* Uninstall the old scanning drivers and install new one that are updated.
* Install all OS updates in your computer.
* Make sure whether you are using wired or wireless connection, the scanner machine should be properly connected with PC system and there must be not any damage in connection.
* Make sure that your scanner or printer is turned on.
* Connect the USB cable to a different USB port on the computer.
* If the USB cable is connected to a USB hub, remove it from the USB hub and connect it to a USB port on the computer.
* Restart the computer.
I wish all above tips will help you in fixing issue, if still scanner gives error then try another method from Canon Pixma MG7720 Manual- https://guideusermanual.com/product-name-pixma-mg7720-manual&po=342426&lang=English