Invitations appear at the top of the Chats column:

• Group invitations, when someone adds you to a group.
• Messages from people your chat privacy doesn't allow to write to you directly: their chat waits here instead of opening by itself.

Click Join to open the chat, or Decline to remove the invitation.

"This chat is no longer available": everyone else has left that chat, so there is nothing to join. The invitation is removed for you automatically; if you want to talk, ask the person to write to you again.