Solution
The most common issue we run into when this occurs is when the modbus ports are already being utilized by another program, or another deployed shared library. Most standard modbus protocols utilize Port 502.
Each time you deploy a Modbus I/O Server it remains running in the background regardless of if you close your vi or project. In order to open up the TCP port for your current Modbus I/O Server you are trying to deploy you'll need to close any previously running servers.
1. You can see which Modbus I/O Servers and/or Shared Libraries that are deployed by using the NI Distributed Manager.

2. Select the previously deployed Modbus I/O Server and click
Stop Process.