Could you please check if there are still Backup Items related to the Recovery Service Vault.
If there are still Back Items you need to stop the Backup first.
After the Backup stopped you can delete the Backup Data. Maybe you have to disable Soft Delete
first, if enabled ( Service Recovery Vault
-> Properties
-> Security Settings
-> Soft Delete
-> Disable
)
When all Backup Data are deleted you should be able to delete the Recovery Service Vault as well.
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Regards
Andreas Baumgarten