This error has to be related with Ethernet communication. In the Modbus TCP/IP driver you need to configure the IP and the port of the Slave device. Probably you have an error in one of this 2 fields. Confirme the IP address of the Slave and the port of the Slave. Normally the port for Modbus TCP is 502.
In this link you have a picture with one of the possibols configuration of the Modbus TCP: http://prntscr.com/i4ttzn.
In this picture I used de port 4001 and not the default port of Modbus TCP 502, this is the only diffrance.