The aggregate backup has failed because an ML2 tape volume containing one or more data sets required by the ABACKUP was not available after two attempts. For a detailed explanation concerning how aggregate backup waits for ML2 volumes to become available and how to modify these conditions, see Tuning DFSMShsm, under the heading Changing the Amount of Time ABACKUP Waits for an ML2 Volume to Become Available in z/OS DFSMShsm Implementation and Customization Guide.