There are two main causes for this happening:
- The phone is running out of memory due to an older model, or having too many other apps open. To fix this, you can restart your device to clear the memory/cache, and the open Mextures to edit as usual.
- You’ll need to allow Mextures to have access to your photos in order to import your images for editing or to save edited images to your camera roll.
To do this go to Settings -> Privacy -> Photos and then select “ON” for Mextures.