From the Microsoft Learn page:
You can't delete a Recovery Services vault with any of the following dependencies:
- You can't delete a vault that contains protected data sources (for example, IaaS VMs, SQL databases, Azure file shares).
- You can't delete a vault that contains backup data. Once backup data is deleted, it will go into the soft deleted state.
- You can't delete a vault that contains backup data in the soft deleted state.
- You can't delete a vault that has registered storage accounts.
Please only delete the backups if you are 100% sure you don't need them anymore.
Here is a guide on how to do it:
https://video2.skills-academy.com/en-us/azure/backup/backup-azure-delete-vault?tabs=portal