I often see people on various servers (pretty much on a weekly basis) wondering what that error means and how to fix it, in which I always reply with :

> Open the "Network ID import/export" utility and fix the Network ID errors.

That error prevents worlds to be uploaded and also prevents play-mode to work.

It would be great if the error message actually explains how to fix that issue. The current error message is really confusing especially for less experienced creators and creators who aren't part of any Discord servers related to world development.