The following key points will give you more information about App Service backup:

• Ensure that the storage account in which you are going to store your backup is in the same subscription as the App Service. At the time of writing, it only supports the general-purpose storage account v1 as a storage account type.
• The backup size should not exceed 10 GB of the application and its database (if the database is backed up too). Otherwise, an error will occur.
• You cannot back up an SSL-enabled Azure MySQL database.
• Do not use firewall-enabled storage accounts to store the backup of your apps.
• If you want to back up the database of an application, but you cannot find it displayed under the Backup Database, ensure that its connection string has been added in the application settings.