The geometry column of the mosaic dataset or the raster catalog you are trying to add rasters to is currently in load-only I/O mode. The Add Rasters To Mosaic Dataset tool and the Workspace To Raster Catalog tool will convert the geometry column to load-only mode if the raster catalog or mosaic dataset was empty. Once the first tool has finished loading, it will return the geometry column to normal I/O mode and re-create its spatial index. Once the geometry column is back in normal I/O mode, multiple tools can be run simultaneously to load the rasters.
Wait until the first tool has completed loading before starting subsequent tools.