During normal operations a MANUAL library will prompt the user for the desired volume name if the volume does not have a valid label and the AUTOLABEL parameter is turned on. However, the utility mode of DUMPDB cannot prompt the user for a desired volume name.
The server unloads the volume and asks for a new scratch volume with a valid label.
The user needs to insert scratch volumes with valid labels or reissue the DUMPDB command and specify the desired volume name on the command line with the VOLUMENAMES parameter. See the DUMPDB documentation.