Upload error may appear due to many reasons. Some are mentioned below.
- If uploaded file is not an image. It only support JPG/PNG/GIF images.
- You have not assigned any default user at UPG Settings. Click on [Save Changes] button to make the changes.
- The logged in user don't have permission to upload media files. Which can be restricted by 3rd party plugins.
- The upload media folder is not writable. You can check it from UPG > Help / System Check.