OMNeT++: "Error in module (cModule) (id=1) during network setup..." Solution

Problem:

Frame title: Confirm
Frame content:
Error in module (cModule) "network name in ned file" (id=1) during network setup: Class "class name in cc file and simple name in ned file" not found -- perhaps its code was not linked in, or the class wasn't registered with Register_Class(), or in the case of modules and channels, with Define_Module()/Define_Channel().

Solution:

Right-click the project > select Build Project
In the console frame, you should get the following successful message: 12:34:56 Build Finished (took 5s.864ms)

Right-click the project > Run As > OMNeT++ Simulation