I just upgraded from Android 10 to Android 11 yesterday (using the factory image, not the OTA image). At the start of the process, I went to uninstall Magisk and received the same error about stock images not existing. I just selected the option to do a full uninstall, then rebooted, and everything was fine.Does anyone know how to fix a "Stock backup does not exist" message from Magisk? I followed the instructions in the OP to update to Android 11 but when I clicked Restore Images that popped up. Worse case I'll do a full uninstall of Magisk, take the OTA update, then reinstall Magisk and root Android 11.
Any help would be appreciated.