What does the message View only. Only the assigned agent can join a live conversation mean?
Only the assigned agent can message the customer. To reply to the customer, assign the ticket to you by clicking Take it in the Assignee field.
Select Submit as Open to apply the change to the ticket.
If you want to join the conversation, you can reply to the ticket by switching to Internal note from the composer.
For more information, see this article: Receiving and sending messages in the Zendesk Agent Workspace.