@Geoffrey37 I was a guest last week. When I first made my reservation, the check in instructions were fairly prominent on my reservation page. But then, the interface changed. Finding my reservation page became more difficult as AirBNB filled the primary page with "Experiences" that they were trying to sell me. Then the check in instructions simply disappeared, a new policy since some hosts were finding that people had the check in instructions and then canceled. When the check in instructions finally reappeared, again, they were hard to find.
I have never used this feature of the app since I prefer to use the messaging system to send individualized messages, and to be honest, don't want to have this important part of my guests' stay controled by AirBNB.
While AirBNB folls around with this feature, I would suggest that you have a saved message ready to fire off when your guests have this problem, don't have the app on their phone, or are just plain flakey.