It is possible you have started using the app with only a single device, and later decided to buy a second one. If this is the case you will need to go the support menu at the bottom right of the App
On this screen you can scroll down until you see Device information
Select the Left or Right LifeEar, depending on which side you have recently bought.
Next click on the “Setup Left/Right Device” button and follow the instructions in the App.