Erro na Requisição com RestTemplate

Boa tarde! Estou com essa mensagem de erro quando vou enviar a requisição para o link da API apresentado na aula ( "http://book-payment.herokuapp.com/payment").

Alguém sabe o que pode ser?

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.web.client.ResourceAccessException: I/O error on POST request for "http://book-payment.herokuapp.com/payment":Connection timed out: connect; nested exception is java.net.ConnectException: Connection timed out: connect
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:973)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)