Is there a way to send a message to participants in an activity based on their completion status?
I would like to be able to send a message to all participants that have not completed an activity.
Not exactly, but you can do this:
This isn't exactly the same as completion criteria, but it will let you send a message to all students who can't have completed the activity because they haven't submitted anything to it. (If viewing the activity is sufficient, e.g. it's a required link, then next to "Show Actions" select "View."
I hope this helps!