If anyone else stumbles across this thread, it's the access code from PearsonVUE that causes this error. When you run a network test the access code authenticates, but then it will expire and give you a new code. The problem being that PearsonVUE gives you no information about this and you end up on a wild goose chase trying to figure out how to allow TCP access.
Just go back to the registration email and follow the steps again. You'll see a new code when you go through the system test and then the network test will pass. Honestly, the amount of forums and Reddit threads that come up when you google the error is astounding. PearsonVUE is absolutely hopeless with their support and documentation, and so many people have missed their exam because of this issue too.