Your Stages SB20 smart bike can be paired directly to Zwift as a Power Source, Cadence, and Controllable sensor. Your Stages Bike does, however, also have two independent power sensors, of which you can pair to the left power meter if you'd like to connect to the power meter directly (or bypass controllable and just use power to ride Zwift). 

See our detailed article Recommended Settings - Zwift for an explanation of both possible recommended pairing configurations and an explanation of how to use Zwift in conjunction with the Stages Cycling app.