Available with Standard or Advanced license.
The overview of the mosaic dataset item may not have any participating raster datasets.
An orphaned overview exists where no mosaic dataset item exists. This usually occurs when you remove a mosaic dataset item, but the overviews were not removed.
It is best to remove the overviews, since it does not reflect the true state of the mosaic dataset. Use the Remove Rasters from Mosaic Dataset tool, with the orphaned overviews selected. You may need to rebuild overviews afterwards.