Simple error -- I selected channels 0:9 for my analog input. Since the USB-6259 does not support "Differential" measurements, analog inputs 8:15 are not available. I reconfigured the analog input virtual channel to read from channels "Dev1/ai0:7, Dev1/ai16:17". Problem solved. I guess reading the channel labels on the DAQ might have been a good start instead of assuming that all channels were available...
🙂 Modified VI is attached in case anyone else has the same issue.