If your on-board computer has not yet been connected to your account, go to "Add new device". Make sure the right device has been selected. Other on-board computers can be seen by swiping to the right. Follow the steps on the app. Once you have selected the right device, make sure that Bluetooth is activated on both devices. Then follow the steps on the app.
If the app already knows your on-board computer, make sure that Bluetooth is activated. Check under "My eBike" to see if a connection exists.