Ensure you are logging in / using the correct account.
Each device can only be registered to a single account. If your device is registered to another user, request the current user log in and delete the device so you can become the primary user / owner. (If you are attempting to add multiple users to a device, please see the " Using the First Alert App" FAQ for details on multiple users / shared devices)
If you are currently controlling your device via the Honeywell Home / Resideo mobile app, the device will need to be unregistered / deleted from that app before it can be paired with the First Alert app. Login to the Honeywell Home / Resideo app, select the device, select settings cog wheel (upper right), select the [device type] Configuration, then select Delete [device type]
If you do not know the current user, or believe you received this error incorrectly, please Contact First Alert / Resideo Customer Experience (see the Help and Support tab within the First Alert app menu)