Hi Pradeep Khantwal,
You need to grant permissionto you keyvault to backup service. If you have the backup and keyvault configuration by default , that's meaning with backup system assgined identity and Key vault with access policies enable you can solve in this way:
- Go to Azure portal > Key vaults > your vault > Access policies.
- Click “Backup Management Service”.
- For Secret permissions, select Get, List, Backup, Restore.
- Click OK, then Save.
If you have a custom configuration on you azure backup and Keyvault you might need to ensure Azure Backup Service has the right role (Backup Contributor)
Let me know if this help you, Luis