Hi Duv1dev1-7278,
We are sorry to hear you are having trouble deleting your app service plan. There are two things you can try.
1.Use the Remove-AzureRmAppServicePlan
PowerShell cmdlet to remove your Azure App Service plan. For example:
Remove-AzureRmAppServicePlan
-ResourceGroupName "Default-Web-WestUS" -Name "ContosoASP"
This command removes the Azure App Service plan named ContosoASP that belongs to the resource group named Default-Web-WestUS.
2.You can try to delete the attached Resource Group and then the App Service Plan should delete automatically. Only delete the resource group if you don't need the additional resources that are in it. To confirm, go to All Resource tab and you should not see that App Service Plan there.
Hope that helps. Please let us know if the issue persists or if you have further questions.
Thanks,
Grace