"This person is unavailable on Messenger" messages are usually an indicator that they have either deactivated their Facebook account or blocked you; sometimes even deleted their entire profile completely from Facebook. To properly reach out to this individual again, it's crucial that you understand why this occurs so you can take appropriate measures to reach out.
Another cause of this error can be when your friend deletes their phone number from Messenger app without your knowledge or has accidentally blocked you; in which case you must communicate with them through an alternative means.
Finally, your messaging app may have become unstable or has an error. To determine whether other users are experiencing similar problems online, check Downdetector for live outage maps crowdsourced by other users.
One simple solution to this problem is clearing your app's cache, which will eliminate any outdated information causing its malfunction. To do this, navigate to your device settings and select Manage apps; from here you can view a list of all the apps installed and check if there are any updates for Messenger that need installing before trying again to send messages to specific people.