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Press and hold the Power and Volume down buttons simultaneously—this will perform a soft restart.
This is what I found online:
"Hold down on an empty home space to access wallpaper settings, go to "Themes," and then go to "Wallpapers" in the shop. You can download a random wallpaper, set it as your lock/home screen, and then change your wallpaper. Also, I'm not quite sure what kind of Samsung this is, but if we received the same error message, then I assume this will work."
"I had the same message pop up. I fixed it by converting my pictures to PNG format (they were in JPG)."
Go to gallery, click on an image, click set as wallpaper
Do a clear cache partition.
Plug into a PC/laptop/USB-C laptop.
1. Turn off the phone.
2. Press and hold the Volume up and Side buttons.
3. Release the Side button when the phone turns on.
4. Press and hold the Volume up button until the phone enters Recovery Mode.
5. Use the Volume buttons to select Wipe cache partition.
6. Press the Power button to confirm.
7. Use the Volume buttons to select Yes.
8. Press the Power button to confirm. Exit.
