We are aware of a bug (as of Febrauary 22, 2021) that is affecting saving of certain images. We are locating the bug and will have an update out soon.
There are two main causes of 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.