Hello @Kanapol Terdvikra
You are correct if you already removed the App there may be a small time to wait until DNS name becomes available again.
You can remove webapp from deleted items, which seen after Get-AzDeletedWebApp pwsh command
$deletedWebApp = Get-AzDeletedWebApp -ResourceGroupName <ResourceGroupName> -Name WebApp1
Remove-AzResource -ResourceId $deletedWebApp.ResourceId -Force
After above steps try to create WebApp again
If you still have the issue after deleting WebApp, open Azure Support ticket.