Thanks for reaching out. We are sorry for the trouble with Lightroom mobile (Android). I'd love to help you figure this out.
Could you please share the version of Adobe Lightroom & the android OS on your device?
How much free space do you have on the device? Have you tried to sign out & sign in again?
The masking space is dark. Please try using a different picture or use a freshly imported image.
Please try to sign out & sign in again in the application. If that does not help, please re-install the application & try again from a different internet network like a mobile hotspot or other Wi-Fi.
Also, please ensure you allow all the required permissions when prompted.
Let us know how it goes,