I can’t download photos from 23snaps on to my iPad

When you first installed 23snaps, your iPad would have promoted you to decide whether or not you would allow the 23snaps app to access your photos. If you tapped No at that point, it would mean 23snaps can't save photos to your iPad directly. You can change this by going into the Settings section of your iPad, tapping the Privacy option, then tapping the Photos option. You should see a list of your apps that use photos, including 23snaps. Just make sure the option next to 23snaps is set to On.