Sending a command means sending a command to an Item. You can’t call an Action using send a command. You need to create a Script Action and import the Action sendPushbulletNote and call it with your message.
Unfortunately there isn’t much documentation for how to do that yet. The PaperUI Rules really aren’t yet read for general use. You might find something useful in Experimental Next-Gen Rules Engine Documentation 1 of : Introduction. I think I covered Actions in the fourth posting in the series.