If this happens when trying to submit a new idea, make sure you're adding a label from the right side column - there's also more info in this Spotify Answer
If you see any other error messages when trying to post, we'd recommend searching in the Community for any similar solved threads.
If that doesn't do the trick, let us know by starting a new thread in our Help Boards - just pick the board corresponding to the platform you're using. We'd suggest making sure to add a screenshot of the error message you see so we can take a closer look. Just make sure that no sensitive or private info is displayed.