This is because there are two copies of the Newtonsoft.Json.dll file. Try navigating to the Assets/MultiplayerBattleWizards/Photon Unity Networking/Editor/PhotonNetwork/Newtonsoft.Json.dll file and deleting the Newtonsoft.Json.dll file.
Thanks for the feedback. It's for a small game jam at uni but I am indeed going to continue developing it. Music, sounds and a win screen are certainly going to be added.