i never used an onboard arduino with Linx. Are you running windows in the lattepanda? perhaps something in OS might be blocking direct access. you may need to investigate. also i am not sure if LabVIEW 64 bits allow using LINX, but you need to try.
if you need a faster solution, then VISA is your friend. you can read and write data from/to arduino using a conventional sketch (not LINX firmware in exemple)
Jorge Augusto Pessatto Mondadori, PhD Sistema Fiep CLAD, CLD