You can learn what error 6 means by opening LabVIEW >> help >> explain error.
Warning 6 occurred at an unidentified location.
Possible reasons:
LabVIEW: Generic file I/O error.
or
NI-488: I/O operation aborted.
I suspect that since you are using an GPIB device, the latter message pertains. I suspect your computer is running much faster than the device. Have you considered putting delays in your code to help diagnose the problem?