This error comes up because you may be already running other project using same 8080 port. If you want to run multiple project then, you can go to project setting > configurations > Edit the one you are using and change runtime port to say 8081.
Hope this helps!
Looks like your port 8080 is already in use by some other application.
Either find the application using the port 8080 (mostly tomcat) and stop it or change the port in Mendix modeler.