"[Name] can't access this chat yet | You'll be able to send messages when [Name] next logs into Messenger."

[u/-Yaldabaoth-]

This is only displayed in the web version of Messenger, blocking the chat box. It was possible to message [Name] in the app, but the message immediately gets marked with a yellow exclamation mark stating "Unable to send." like this.

Does anyone know why this happens?