Thank you Richardson.
The website of the link I couldn't open but I think I know what you're suggesting.
Actually, I am not looking for the way to reclaim thin provisioned disk space since I am not bothered by the used space of the datastore.
Since "When a file is deleted from the virtual machine OS the free space is available for the OS", I can't figure out why the OS of the VM consumes less far away from used storage but vCenter still gives the "no more space of the virtual disk" error.