To use SyncBackPro to backup your Google Photos you must first give permission for Google Drive to access your Google Photos. To do this simply go to the Google Drive web interface and give permission. If you are not prompted to give access you can click the gear icon in the top-right and select Settings from the pop-up menu. You then need to enable the option Create a Google Photos folder
Once permission is given you'll be able to access your photos via the Google Photos folder in Google Drive.
The following Google KB article gives more details: