Oops, I edited my last post and I guess you saw the pre-edit post. Here it is again. Once you make the change, you'll have to wire the stop button directly to the loop stopper. Also, you'll want to add a delay in the loop (just anywhere) so it's not looping as fast as possible.
Insert a Build Array node in to that wire. It probably requires an array of data, but you only need a single value, so you need to input an array of just a single element.
Scratch that, it looks like you have your DAQ Assistant set to Continuous mode, which means it's going to try to write a whole waveform with a set sample rate, etc. I don't think you want this if you're just trying to set On or Off. Instead, double-click your DAQ Assistant and set to 1 Sample (On Demand) and it will then take in a boolean array. The array is becuase you might want to write to multiple digital channels, but in your case it's just the one, so it'll be an arraywith one element.
