It is possible the person you are unable to contact might have deactivated their Facebook or Messenger account, or their account has been terminated by Facebook. Since the account no longer exists, Facebook displays this message to indicate the person is no longer available. Therefore, you cannot chat with them. To rule out this possibility, you must ensure the account of the person you're attempting to Whatsapp Number List contact is still available. For that, get in touch with one of your mutual Facebook friends and ask them to check the person's account. Alternatively, you can create an anonymous Facebook profile and search for your friend's account yourself.
If the person's account no longer exists, they have either deactivated their account or got suspended. If they have deactivated it, you can only contact them if they reactivate it. If Facebook has banned their account, the user may have to create a new account.
Can you recall a harsh conversation with the person for whom this error appears? If so, your guess is correct—the person has blocked you. When someone blocks another user, Facebook restricts all communication with them and displays a "This person is unavailable on Messenger" message.
To confirm this, use the Facebook search bar to find your friend's account. If the account doesn't appear in your search results, but it does in your mutual friend's searches, you have been blocked. We have an article explaining several other ways to confirm that someone has blocked you on Facebook.
If you're blocked, there isn't much you can do until the person unblocks you. Once the person unblocks you, this error in your inbox will automatically disappear.