You'll get that error if you're trying to communicate with the USB VNA chassis that lists under the "PXI" group in the IO Libraries UI.
First, install the USB VNA app if you don't have it already. The USB VNA firmware is available here: PXIe/USB Vector Network Analyzer Firmware Update (64 bit) | Keysight. Then run it following instructions given in the link of hognala's post on the PC that this USB VNA is connected. This will launch the USB VNA's firmware and make the instrument available for use. For the USB VNAs of the P5xxxA line the SFP is really the instrument's firmware, so that has to be running for the instrument to be an instrument.
Once that is done you should see a new instrument under the "LAN" section of IO libraries that is a HiSlip instrument. If not, do a rescan in IO libs. You should see something similar to what I show in my screen grab below (ignore the obfuscated parts for the obvious reasons). That's the instrument you want to connect to and the address you want to use in your programs.
I think you need to see this documentation page about running the SFP: http://na.support.keysight.com/pxi/help/latest/S0_Start/Launcher.htm#How_to_start_the_SFP_Launcher