AZURE BL 1-4 Storage Accounts data at rest must be encrypted

Storage Accounts are encrypted by default with Microsoft-managed keys. This check ensures that the default encryption is not changed or interfered with.

For remediating from the Azure Portal 
  1. Goto Storage Accounts.
  2. Select the storage account you want to remediate
  3. Click Encryption.
  4. Set type to Customer-managed keys.
  5. Use option Select from the key vault or Enter key URI to set up encryption with your customer key.
  6. Save the setting before exit

You can read more about data encryption in Azure here.