You can cancel pending SLP operations by referencing the Lifecycle Name OR Backup ID.
nbstlutil cancel -backupid <backupid>
nbstlutil cancel -lifecycle <lifecycle>
If you cancel by referencing the Lifecycle Name, it will cancel all pending SLP operations referencing that SLP. If you cancel by referencing the Backup IP, it will only cancel the pending SLP operations associated with that backup.
SLP Cheat Sheet: https://www.veritas.com/support/en_US/article.000013928
How to find Backup ID: https://www.veritas.com/support/en_US/article.000026300