Stages Cycling app is compatible with all generations of Stages Power meter, however, it is not capable of upgrading legacy firmware on a Gen 1 or Gen 2 power meter. Due to changes to previous firmware versions, power meters on older versions of firmware will not successfully connect to the app.
Minimum Gen 1/Gen 2 firmware: Version 2.0.81.
To fix this problem, install or open the StagesPower app. Connect your power meter and update to firmware version 2.0.86. Once updated, you can return to the Stages Cycling app and pair it as a new device under the Devices menu.