The red triangle appears when there has been an ePub export error at some point. This could be due to a loss of internet connection or an error in the document that prevents the ePub export. Rest assured that this icon will disappear once you successfully export the ePub again.
If you encounter an error popup while exporting the ePub, our troubleshooting page can be helpful in identifying possible issues within the file: https://www.atticus.io/troubleshooting-export-errors/.