Was the TLServer still running when you try to transfer program to the PLC? TLServer must be running (although you can minimize it) so that the TRiLOGI software can communicate with the TLServer via TCP/IP and TLServer then connects to the PLC via RS232 or RS485.
You can also try "Controller-> Disconnect" to close the current connection first then re-try the connection to the TLServer. Sometime it may be because the previous connection was lost but not properly closed.