NI 6341 buffer problem

We have a NI USB 6341 DAQ. I want to output an analog sawtooth waveform by external triggers. In other words, there will be a sawtooth signal generated every 480 input pulses. The frequency of the trigger is 5k Hz, therefore, the generated sawtooth signal is around 10Hz.

But I keep on getting error codes about buffers. This time, the error message is:

Error -201025 occurred at DAQmx Start Task.vi:7220001

Possible reason(s):

Non-buffered hardware-timed operations are not supported for this device and Channel Type.

Set the Buffer Size to greater than 0, do not configure Sample Clock timing, or set Sample Timing Type to On Demand.

Task Name: _unnamedTask<856>

Source:

DAQmx Start Task.vi:7220001<append>
<B>Task Name: </B>_unnamedTask<856>

Can anybody help me with the code attached?

Thanks!