Is this on iOS or Android? If this is Lightroom Android, is it possible you changed the setting for location where photos are saved to use your SD Card? Other users report this error when doing that, and changing settings to save photos on device memory again fixes the problem.
I think you may have the same issue as Alessia. If you use Lightroom Android, and changed settings on your device to save images to your SD card, pleas try changing settings back to save to device memory instead. Does that help?