@Cream Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
I understand that you are unable to delete the Azure AI service account, and you are facing the issues.
If you are facing the above error using the portal I would suggest you try deleting the AI Service Account using either the below PowerShell or AzCLI Cmdlets.
PowerShell:
Remove-AzCognitiveServicesAccount -ResourceGroupName <AIServiceAccountResourceGroupName> -name <AIServiceAccountName>
Azure CLI Cmdlet:
az cognitiveservices account delete --name <AIServiceAccountName> --resource-group <AIServiceAccountResourceGroupName>
Refer to the cmdlet's documentation, Remove-AzCognitiveServicesAccount & az cognitiveservices account delete.
Hope this helps, let me know if you have any further questions on this.