@Clair27
Yes...unfortunately Airbnb made a software change and now tie the check-in instructions link to the individual reservation number instead of the Arrival Guide. Too bad, as many Hosts and guests found the link very helpful. See this previous thread:
https://community.withairbnb.com/t5/Ask-about-your-listing/Check-in-Instructions-Link-Not-Working-Pr...
Airbnb even had guests pointed to the wrong place in their messages to guests "Things to Know" instead of "How to Get Inside" but have since corrected that. Some Hosts are using the Help Article now since it was corrected and using the Help Article Link:
Where to Find Your Check-In Instructions
https://www.airbnb.com/help/article/100
Hosts have been asking for a message shortcut for a long time that links directly to Check-in Instructions to no avail. Suggest you add your own feedback to that list if you like.