There are a few possibilities, and the specific error message will tell you more:
Do you see a message about your Jetpack connection?
If so, run your site through the Jetpack Debug tool and see if there are error messages. Follow the troubleshooting tips there to get Jetpack reconnected on your site, and that should resolve the error.
Did the WordPress App begin upload and then provide a ‘Failed’ message?
Try uploading a smaller image size. Also, check that your device has enough storage space available (we recommend at least 1GB of free space)