If your FitBit application requires running on port 8080, you will run into exactly the problem you have because MagicMirror also runs on that same port. You will have to change one of them. MagicMirror is easy to change, it’s in the config.js file. Change the port setting to something else, like 8081 …or 8585 … or 8123 … doesn’t matter, as long as it’s above 8000, and not 8080 since the Fitbit runs on that port.