First, restart the Scout pairing mode by connecting a charger, and force close the Find My app and reopen it. Check to see whether the Scout can now be added in the Find My app.
If the above does not work, your Scout has probably made an incomplete connection to the Apple Find My network. You will need to perform a factory reset to restore the connection. First, ensure that the Scout is disarmed and then remove the Scout from the Knog app. Next, forget the device from you phones Bluetooth device list and then perform the factory reset and set the Scout up again following the set up instructions closely.
You can find the factory reset instructions for Scout Bike here, and for Scout Travel here.
Do not attempt a factory reset while the Scout is in an armed state.