in addition to Olaf's answer:
You have to get the Recovery Services Vault object in the variable $vault
first:
$vault = Get-AzRecoveryServicesVault -Name "TestVault-rsv"
Remove-AzRecoveryServicesVault -Vault $vault
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten