If you get the "No user slices" error while exporting your designs from Photoshop to Zeplin, this might be related to Photoshop's "Slices" settings. Following these steps should help:
Draw a slice on your artboard.
Select "Save for Web (Legacy)…" from the menu "File > Export" up top and click "Save".
Select the "All Slices" option and click "Save".
After following these steps, you can delete the slice you just created, and you should now be able to export the file.
If the issue happens again, you can contact us at support@zeplin.io.