There's a known bug with USB Serial convertors.
It's supposedly fixed in VISA 15.0 but I haven't had a chance to test it.
The user reported that the manufacturer-supplied application also does not work. Have you tried swapping out the USB to Serial converter?
If you can't get the manufacturer's software to work, then that is proof this isn't even a LabVIEW problem. You need to go back to the mfgr and work with them until you can get their software to work. Maybe the problem isn't even the USB/serial adapter, but their device is broken.
So, it means that there is nothing wrong with your LabVIEW work. You should contact the manufacturer.