This is a comon error when trying to action a backup in recovery services vault in azure using a non supported disk or storage that Azure vault cant supportin your tenant .
In order to resolve you will have to check the support matrix and check the limitations or how you configure the vault in order to accomplish your backup
In my case I was trying to backup a Zone redundant Premium SSD wich was not support for our Vault .
MS Links for Azure Backup
https://docs.microsoft.com/en-us/azure/backup/backup-create-rs-vault#set-storage-redundancy
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