Solution
Not all M and X Series devices support analog triggering on all channels. To determine what physical channels can be used as analog triggers, see the specifications document for your specific M Series or X Series device. You can find the specifications document for your device by searching for your device model number at NI Product Manuals.
To use analog triggering, change the polymorphic selector on the DAQmx Start Trigger vi to 'Start Analog Edge', as shown in the snippet below:

Notice that by default the source terminal of the VI is a string data type instead of a DAQmx physical channel data type. Consequently, you will have two options:
- Create a DAQmx physical channel constant or control and wire it into the source terminal to be coerced to a string, and select the physical channel to trigger off of from the dropdown menu.
- Type out the correct name for the specific physical channel that you would like to use into a string constant or control.