If sleep mode is enforced during a backup job, the job would fail.
If your computer is in the standby mode at the time when the backup job starts, Veeam Agent for Microsoft Windows automatically wakes your computer from sleep. More information can be found here.
However, the sleep mode can be enforced by third-party applications. If this happens while a backup job is running, the affected job would fail. When the computer returns from the sleep mode to normal operation, the backup job will be retried automatically.
If it is mandatory that the backup jobs complete as scheduled, you will need to manage the operation of other software so that it does not interfere with the operation of Veeam Agent for Microsoft Windows. You will need to ensure that a sleep mode, shutdown or reboot will not be called during a scheduled backup job.