OnTLSNotAvailable is a Published property in TIdFTP that represents the event handler signalled when the client has failed to provide implied support for TLS (Transport Layer Security). 

OnTLSNotAvailable allows the application to determine the action to take when a TLS negotiation has failed based on the value used in UseTLS. Arguments to the event handler include the FTP client for the event notification, and a boolean argument that indicates if the process should be allowed to continue. Applications can set the value of the boolean argument in the event handler procedure.