The Echelon Fit app will only take a photo of the Echelon EX3 bike's serial number.
If you have the Echelon Connect Sport model, you need to select the right equipment:
...MORE/Select Equipment/+/Echelon Sport
This is stop the loop of 'take a picture of the serial number'
If you are unable to take a picture of your EX3 serial number this may be due to a printing issue with the label on the reverse of your EX3 console.
Scan this image instead: