According to the Facebook developer page, receiving the "message not sent: This person isn't available right now" notification is categorized as a permission error. This notification could indicate that the intended recipient has chosen not to receive your messages by blocking you on Facebook or that their account is no longer active on the platform.
Determining the exact reason and drawing a definitive conclusion can be challenging. Here are some possible explanations for encountering the "person is not available on Messenger" error message:
1. The person has blocked you.
2. You have blocked the person.
3. The person has deactivated their Facebook account.
4. Facebook has suspended the person's account.
5. The person's profile is set to private.
6. The person has deleted their Facebook account.
7. The person does not have the Messenger app installed on their phone.
There are also some solutions to potentially fixing the person unavailable on messenger issue.