Document SAS expiration policy for Azure Files Backup

Added information about SAS expiration requirements for Azure Files Backup.

Author: Charmila1

articles/backup/troubleshoot-azure-files.md

@@ -325,6 +325,16 @@ Recommended Actions: Ensure that the following configurations in the storage acc
 
 **Cause**: Storage Accounts with [Smart Tier(Preview)](../storage/blobs/access-tiers-smart.md) set are currently not supported.
 
+### UserErrorStorageAccountSasPolicyNotSupported - Azure Files Backup requires a minimum SAS expiration period of 8 hours
+
+**Error code**: `UserErrorStorageAccountSasPolicyNotSupported`
+
+**Error message**: Azure File Share Backup requires a minimum SAS expiration period of 8 hours.
+
+**Recommended action**: 
+- If the SAS token expiry is shorter than 8 hours, the Backup service cannot renew the token in time. This leads to authentication failures and can interrupt ongoing backup or restore operations.
+- Set the SAS expiry upper limit to 8 hours or more to ensure the Backup service can successfully renew the SAS token and maintain uninterrupted backup and restore activities.
+
 ## Common policy modification errors
 
 ### BMSUserErrorConflictingProtectionOperation- Another configure protection operation is in progress for this item