Generally Add Watermark needs following permissions to be granted on your device:
- WRITE_EXTERNAL_STORAGE: Allows an application to read from and write to device or external storage.
- WAKE_LOCK: Allows to keep device from sleeping during processing of multiple images.
Beginning in Android 6.0, users grant permissions to apps while the app is running, not when they install the app.
When you start the app for the first time, you should see a permission request dialog:
Once allowed it doesn’t appear any more. But when you for some reason don’t allow, the app will generate errors when trying to load a source image or save processed image. In this case just restart the app, it will show the permission request again. Press “Allow” and that’s it!
Sometimes the permission request doesn’t appear, instead you see a “Screen overlay detected” message. This is because of another app with a floating overlay like Facebook Messenger, Twilight, CleanMaster, or Lux, that blocks permission requests from other apps. In this case please try to close all such floating activities then launch Add Watermark again. It is supposed to request permission without the “Screen overlay detected” error popping up.
If it didn’t help, follow this step-by-step guide:
https://www.androidpit.com/how-to-fix-screen-overlay-detected-error
Add Watermark for Android 