It's not an error, it's a warning. Usually it happens if you import a different bot. For example, you create a bot and upload 10 documents. Those documents are stored in the documents' database. If you then upload a different bot, the documents don't exist in the bot anymore, but they are still in the documents' database and your published bot will still answer using those documents. Clicking the button essentially means "Please go ahead and synchronize my documents". It's just to warn you before deleting your documents. If you click Remove documents, it will impact the bot's responses, even if it was not published.