"Project Converter - A project converter must be installed before you can convert to or from NVivo for Mac projects"
If you are using NVivo for Windows with Collaboration Cloud and receive this error while synching, please check for updates and install the latest update.
Once you update you should be able to download the converter. Collaboration Cloud in NVivo 14 and NVivo 15 requires the converter to be downloaded even if no project members use Mac. This is to allow Mac users if necessary in the future. There is no way to bypass the converter.