Why does the canned responses list say "No canned responses for this ticket's group"?

The picker only offers responses that fit the ticket: responses with no groups (offered to everyone) and responses limited to the group the ticket belongs to. A ticket without a group only gets the responses offered to everyone. If a response you expect is missing, ask an owner, admin or the group's admin to add the ticket's group to it under Configurations > Canned responses, or to clear its groups so it is offered to everyone.